You have to decide what it is you are trying to do. Are you trying to reproduce your favorite pickup? Are you trying to design a pickup for a special instrument? Are you trying to come up with something new and innovative? Are you trying to reproduce a vintage pickup? etc...
If you are trying to reproduce a pickup… vintage or not… you need to hunt down the specifications of that particular pickup. Search around a bit on the Internet, or check out some or the pickup maker forums. More than likely someone knows about you pickup, and can help you with the details.
If you are designing for a special instrument you need to come up with a list of expectations you have of this instrument. .... Work in progress!!