A resistor alone is not sufficient, unless your circuit has a constant and known current draw. Recall that V = IR, so you can calculate the voltage drop (V) across a resistor R only if you know the current I through the resistor.

If you are needing some "standard" voltage output (like 3.4V) you're better off using a three-pin voltage regulator with a heat sink.

Well, do you know the supply current? If so, use R = V/I (where V is the voltage you want, and I is the supply current) work it out and that should give you the desired resistor value.

If you are needing some "standard" voltage output (like 3.4V) you're better off using a three-pin voltage regulator with a heat sink.