You need to specific the current of your application. Ohm's law is V = IR. If you know any two of those quantities, simple algebra gives you the third. In your example, you are asking for R = V/I, where "V" is the voltage difference (19-12 = 7, 12-5 = 7, etc.). If you know I, then you can calculate R.

If your current load is not constant, then you should be using a voltage regulator, not a resistor.

That depends on how much current you're drawing, which depends on resistance elsewhere in the circuit. (And/or impedance, if you're dealing with alternating or pulsed current rather than DC.) If you know those other numbers, you can calculate this one. If you don't, you can determine it experimentally... but then if they change for some reason, the voltage will change too. A better solution is to use active regulator circuitry, which balances this dynamically and which wastes less energy in the process of doing so.

active| newest | oldestdifference(19-12 = 7, 12-5 = 7, etc.). If you know I, then you can calculate R.If your current load is not constant, then you should be using a voltage regulator, not a resistor.