I have a pile of DMMs, the are cheap, but good enough. Somewhere, I may still have a nice meter (you know, the kind with a needle resting on some diamond chips to pivot smooth and effortlessly. I cant remember the last time I saw it, but its a basic VOM.
I think i understand the basics of the calculations, but it would be nice to understand why the parts are needed. I also dont like the guessing part of the calculation, it seems to me that the part will likely behave about the same under similar conditions for most people. It seems like it should be easy to look that up in a chart or graph, and get close with your calculations, but I wasnt able to do that, I had to test things, before I could get in the ballpark.
The last several posts, I've only been talking about using 2n2222 BJT in my circumstances of lighting up to 0 to 4 RGB LEDs (0 to 12 LEDs total) at one time.
The posts with the schematics show you how the transistors are being used. That same circuit can probably be modified to use mosfets, or ULN (darlington arrays) chips easily though. I did test all 3 types of parts using the same LEDs and resistors that I used on the cube.
I do also have unfinished projects that will need mosfets, and I've picked out some tiny parts that hopefully will be suitable for my needs (about 1 amp), and as I understand it, they dont need a resistor between the arduino, but they need one between the gate/source, I think I have some 4.7k's for that.