Calculating the resistor for a 12V pushbutton
The actual switch rating is not important, as long as it's high enough to cope with what you're doing, which it is. You should NOT use the PWR pin for this, as the raspi inputs work on voltages 0V-3.3V, and are NOT 5V tolerant. You're sort of confusing two different circuits here. Normally, we would use this circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

In this case, R1 sets (pulls up) the voltage at the GPIO when the switch isn't pressed. When the switch is pressed, the voltage at the GPIO is 0, which means there is a current flowing through the resistor and the switch to ground. You want this current to be fairly low, well below the rating of the switch. It doesn't have to be at all precise, so a 10k resistor is usually used (I say usually because that's what I always use, and it's a common value when you just need a medium value resistor). This means that the current flowing through the switch will be 0.33mA.

However

The raspberry pi actually has built in resistors, so you don't need to add an external resistor. The circuit is as follows:







simulate this circuit

You can also, if you want, use the internal pull down resistors, and connect your switch between 3.3V and the GPIO. I can't really think of a reason to do one over the other, except that a lot of chips only have internal pull-ups, and ground is usually pretty accessible in a circuit, so it's easier to use pull-ups and gnd.