Most USB use a ferrite bead on the supply so I would say that's what your looking at It's not going to hurt to use one there.

The Op is basing  this on the total leds looking like 10 ohm load 30 volt 3 amp 10 ohm.

You set it as the leds came on  And it worked it will for now but it may not on the next set.

I played with current driving leds and the voltage idea your using.

I did it like this I hooked 10 red leds in series I supplied 1.5 to 25 volts as I turn the pot led 1 came on
the supply used  lm317   turn it up led 9 came on went for led 10 and on it came.

I sat there and said cool till they popped like popcorn but my supply could put out 3 amps your is limiting the current.  

Basically, I am using the source-drain resistance of the MOSFET as a current-sense resistor. A crude measurement of the current is all that is necessary.

That's not a real good idea you could read the copper trace resistance would work better and save you a headache

That's the way cheap DVM read it better one's use a wire thats made to have constant reading with temperature.

I would limit current to under the clamping diodes spec. At the 48 volts

There two ways to use the  vinyl . First one you buy a $400.00 dollar cutter. http://www.instructables.com/id/Fast-and-Easy-PCB-Prototyping-with-Vinyl/

And the one tho OP is taking about the vinyl paper that keeps it from sicking you remove it print on it place the vinyl back on and iron it peel the vinyl place on copper and iron it on then just peel it off and etch. I been looking for the second way can't find the link. http://en.electroni-city.com/

Is this in parts or what I've seen then cut the vinyl and use that I also seen something like the OP is taking about You pull the   vinyl  copy to the backing and they ironed  it back to  the vinyl  then put the vinyl on the copper and ironed that on.

But I have not tried that But I do have the vinyl and I'm going to try this this weekend I wish the OP will post a link.

 fungus I was just adding to what you where saying.  In all reality  A  relay be it Solid State or  mechanical. Would give you better heating.

@ what fungus said your power loss is more with the P channel then the N channel.
And like fungus  said you can't use just a P channel MOSFET you have to add a driver.

Power flows from  negative to positive

negative charged electrons move toward the positive

So you are cutting off the Power using a N channel

And a a 1 inch piece of 18 gauge wire would  read 10 Millivolts @12 volts 10 amp and and put out 100 Milliwatts of heat I'm sure it's not going to get hot you'll have to scale it with a opamp.

I have a DVM it's not a cheapo and it reads across a 3 inch piece of wire it's 14 gauge   

Yes there some really good stuff on that site how the zener set's the time it take's to close the injector.

This is done by using a zener diode across the PCM driver (transistor or MOSFET).  See Figure 6.
As the magnetic field falls back into the injector winding the energy is allowed to loop through the circuit.  This allows the current to diminish at a set rate.  The lower the voltage is set by the zener diode, the more energy is allowed to loop through the circuit.  If a diode were used rather than a zener diode, this would let the most energy allowed loop through the circuit.
A diode will allow the stored energy to loop until it reaches source voltage, in this case 12 volts.  This would allow the injector the longest period of time to close.  The higher the zener diode voltage, the shorter the period of time allowed for the injector to close.  This is due to the energy looping through the circuit being cut off early by the voltage rating of the zener diode.
If a 65 volt zener diode is used, the energy that is looping through the circuit is stopped at 65 volts which is 53 volts sooner than a diode, that allows the energy to continue to loop through the circuit until it reaches 12 volts.  So the energy from a higher rated zener diode will shut off the energy looping through the circuit sooner which allows a faster pintle closing rate.  Likewise, the energy from a lower rated zener diode will allow the energy to loop longer which will cause a slower pintle closing rate.  This rate is set by the zener diode voltage which is matched to the injector design.
The delay in closing voltage can be seen in Figure 2 Part H on page 27.  Once the Pintle starts to fall through the magnetic

I think that the shunt resistor is just a waste of energy

You don't want a p channel you want a N channel  MOSFET Like this one  COM-10213 http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/General/FQP30N06L.pdf

And you don't use a shunt resistor that waste amps you use a small one .001 ohm I wouldn't buy one I would make it with a piece of wire   

http://automotivetestsolutions.com/images/escope/fuelingectortutorial/FuelInjectorWaveforms.htm
Use a zener diode

This is not basic 101 there a lot that has to happen here to do this right.

This induced voltage is called the fly back voltage.  The flyback voltage is then clipped (Figure 2 Part G).  This voltage level changes between manufactures and systems.  The flyback voltage is adjusted for the injector design being used in the circuit.  The flyback voltage is set for the electromagnetic coupling of the injector and the mechanical spring rate.  The magnetic field around the injector winding is stored energy which is used to control the speed that the pintle is closing with.  If the pintle is allowed to close too fast the pintle and seat will become pounded out and will begin to leak fuel.  With a fast closing rate the pintle can also bounce causing extra fuel to be delivered to the engine.  This extra fuel cannot be controlled accurately so the engineer must adjust the energy held within the flyback voltage to accurately control the closing rate of the injector.

I hooked the board as in the picture I posted in post 2 I used the sketch that comes with the arduino ide ver 1.0.3  windows 7

It's just sat there and then it would pop up a error that I posted changed to my old laptop which has ver1.0.2 of the ide and windows XP  and it happly burned all 3 chips .

Code:

// ArduinoISP version 04m3
// Copyright (c) 2008-2011 Randall Bohn
// If you require a license, see
//     http://www.opensource.org/licenses/bsd-license.php
//
// This sketch turns the Arduino into a AVRISP
// using the following arduino pins:
//
// pin name:    not-mega:         mega(1280 and 2560)
// slave reset: 10:               53
// MOSI:        11:               51
// MISO:        12:               50
// SCK:         13:               52

I burned it with no problem on my old computer.

Auto-reset not disabled on the programmer I pulled the chip from the uno and hooked it up on the bread board with 16mhz crystal just as it is on the the uno Im doing it like this
http://www.gammon.com.au/forum/?id=11637