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.