Bridge, probably halfway between current pup and end of bridge. IMO that's where most of your customers will want it. You can just route the neck pup and add the second route if/when you need it, unless you're planning on having someone make complete bodies.
It helps to be as flexible as possible. With a custom business you can use whatever pickup your customer wants...unless you're planning to place a big enough pickup order to get a discount (12 sets from Seymour Duncan, for example, 60% off list when I bought 24 SSBs. That buy in gave me the right to buy onesies of any SD pickup at that discount, except Antiquities).
It's good to know the 'sweet spot' for the different pickups you use if you decide to offer a choice. Your suppliers should be able to supply that info.