My best advice is keep the tips wide initially and forget the rounded belly stuff until it's coming back to about 20", think of it a being rectangular with the corners taken off.
I've not worked much Osage so I won't try to give figures. Don't worry about the 5/8 thing, it has to be pretty thin an wide to miss that.
Also make it an inch longer at each end that you plan for. The extra inch and extra tip width all give you some wriggle room if it starts to try and go sideways on you.
It's not so far away from an what you guys call a longbow, just no deep grip of cut away, the belly is rounded a tad more and it's a bit longer.
I'll bet if you measure some of your bows they will be inside the 5/8 "rule" anyway.
Del