JimH, you mean Ohm's Law?
Basically, Voltage = Current x Resistance...
Resistance, then is Voltage/Current
On the 12V rail, you want a current of at most 15 mA, or 0.015A
Therefore, reistance must be at least 12/0.015 Ohms, or 800 ohms...
For a safe level of 5mA, you'll need a 2,400 ohm resistor
Hope this helps,
Living proof of John Gabriel's theorem