There is no optimum pickup position!
It's a first time body design, but not my first build with this kind of pickups.
The pickup position I'm using here with dual coil pickups was determined when I build my previous six string bass. The center of the bridge coil is roughly on the same center as the Musicman humbucker bridge coil, relative to this coil I position the neck pickup, the center of the coil closest to the neck is on the same distance that's between the (jazz) pickups of a Warwick Corvette, these two coils are the ones I will use in single coil mode.
These distances have been upscaled to a 35 inch scale by the way.
I really like the sound these positions give me, and then there are also the added sound possibilities with the Series/single/parallel switch for each pickup.