I'm purely an amateur as far a wheelbuilding goes, having built up maybe a half-dozen over the past few years. FWIW, though, I find Spocalc to be a rough guide at best. It gets you in the ballpark, yes, but in more often than not I've ended up going up or down from their number to get it to work. The wheel I'm doing final truing on now, for example, was supposed to use 308 mm spokes, according to Spocalc. (It's a four-cross on a 27 inch rim). But they were too long--I ran out of threading and had spoke sticking through the tops of the nipples before I got up to the required tension. Unlaced the wheel and relaced with 305s, which I fortunately had on hand and was in business.
I don't know, in my experience your chances of nailing the right spoke length on the first try are not great. Maybe others have not had that experience, but it's what I've found. The only good thing about it is that eventually you end of with a stash of not-quite-right spokes that might turn out to be just what you need for the next project.