That's correct, and you're understanding the concept. Twice the power would be a guide number that's about 1.4x larger (square root of two). In the pre-automatic flash days (including manual electronic flashes and flash bulbs), this was one of the ways we set the aperture for a correct exposure. Usually there would be a dial on the flash that you could set to a certain ISO and it would then in effect do the math for you. It still works with today's flashes as well when you're in manual mode. Even if you're using automated flash, it's a helpful concept to understand.