In theory you'd want to move your crankset 7.5mm outwards (assuming that your FD can reach that far) so if you're dealing with a symmetrical BB you'd want a spindle 15mm longer than recommended for a half bike.

If your timing crank allows you to mount the chainring (or is that beltring?) on either side of the spider you can get away with a narrower captain's BB.

What crankset are you using?