It's no different than with any spoke calculation.
Just verify 3 things and you're home free.
1. On hub, be sure to use a diameter at where the head pockets, so it matches the holes for J bend spokes.
2. Measure ERD including the nipples you plan to use. Deduct 1-2 mm ( per side) if you wish to spokes to end shy of the top.
3. Know your calculator. Some expect an ERD measured at the nipple seat and add 3-4mm for the nipple head. Others expect you to enter rhe ERD where you expect the spokes to end.
So, you might need to adjust the result according to the above 3 factors, but that's easy enough if you know what you're doing.
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