When I'm experimenting with a pickup I'll usually start with what is the general consensus for the specifications, and work from there. The hardest part is getting the tone you want, and being balanced between the pickups as well. Manipulating the wind count may allow you to calibrate the pickups to each other. P90's are very powerful pickups. They have a broad flat coil, and two big powerful magnets that are coupled together. They're loud. A Tel, or strat OTOH that has a tall narrow coil, with rod magnets that aren't nearly as strong, less wire, and less input from the string due to it's position closer to the bridge, it might have a hard time competing. Achieving the optimum tone from both pickups without having one of them be significantly louder than the other in this type of circumstance can be difficult to say the least.