newuser:
I have the following cirquit  my goal is to get 4.29V down to less than 3V. I am using online voltage calculator (http://www.ohmslawcalculator.com/voltagedividercalculator). Arduino is powered through ftdi usb from my computer.
 4.29v (Arduino pro mini pin 3 turned on)



R1 (10K)

X expecting 2.86V but get 0.52V

R2 (20K)

ground
Remember Ohm's law????
Compute the current through 30,000 ohms with 4.29 volts. How much current in milliamps?
Now, using Ohms law again, compute the voltage across 20,000 Ohms with the milliamp answer you got for the first computation. Bet it is close to 0.52 volts.
Now, using the Ohms law again, how much current, in milliamps is needed to get 2.86 volts across 20,000 Ohms?
With that last answer, use Ohm's law again to compute the total resistance needed to allow that much current in millamps to pass through the resistors.
Subtract the 20.000 ohms from that answer and you will know the value of the top resistor.
Paul