I'd have to see why . . but last time I checked, for that span, you mark the middle finger, measure 3/8ths up, mark the ring, then put the holes in it . . I would obviously have to think those guys that showed you how to do it know what they're talking about, but I would guarantee you I could drill you a ball fingers first. I drilled a ball perfect the first time for a guy with webbed fingers . . meaning that his middle and ring fingers were fused from the base all the way up to the tips, just getting separation at his cuticles. This was a conventionally drilled ball, so no way to accurately measure full finger size, to know what oval width to go for, what pitches to start at because of course the fingerholes had to overlap, but still have a ridge in between to grip with, and only an older crude way to measure span because of course with his fingers being fused he couldn't properly fit into a measuring ball. Send me your specs and I'll punch up an old junk ball we have just to see. Keep in mind I'm not trying to prove you wrong, because being a pro shop guy, anything I can find to make me better, I'll be all over it, I just really don't see how drilling a thumb first is more effective. And honestly, 98% of the balls I drill have grips in them, and the ones that don't are easier to drill because I don't have to account for the differences in grip wall thickness in laying the ball out.
While I don't echo Russell's abrasive attitude . . I do echo what he says, and actually said most of it earlier in this thread. Fingers first allows you miss room and adjustments, something that thumb first doesn't allow. Again, obviously the people who taught you are established, respected names, I just wonder if it wasn't a trick to your fit specifically . . because I've never had problems nailing any fit before, and certainly nothing that going thumb first would fix. I am however EXTREMELY curious about what exactly they told/taught you, if you wouldn't mind sharing it or at least covering the basic concept.