I wonder why this is. Spocalc uses a geometric formula -- presumably this is not some "fudge factor" thing, but something from common usage or a researched methodology. Why would the bike shop's "professional" software differ in method? Some refinement that Spocalc doesn't include?

I notice that a lot of the rims have (manufacturer's standard) "spoke support" or "nipple seat" plus 3 mm for the nipple head. Is that too much, perhaps? Do different brands of nipple have different head thicknesses?

I've never built a wheel, but I'm screwing up my courage to try it.