I've got the same issues working right now. Trying to tune a 48# longbow and the 44# recurve. The DSC calculator says I should be at 500's on the longbow and 400's on the recurve. Based on the limited knowledge I have, I think 500's all the way around. I asked the same questions as you and received some answers of even a 600 spine possibly. Combine that with hearing about folks with pretty much the same specs as I have and their talking about 145+gr points and 50 or 100gr inserts on a 500 spine out of a 45ish # bow and I'm losing my mind.
I'm determined not to give up and figure this all out. But, like I said earlier as soon as I learn/figure something out, here comes that damn wrench.