Keep the width the same for at least 6" both sides of center. The dimensions you've marked out will give you slightly too much bend in the handle early on.
Your width and thickness are spot on though, for the weight you're after. If you can, try and get the stave down to exactly those numbers (making the center width the same as I mentioned above), get the stave rounded off nicely and then spend a fair while sighting down each limb in turn from every conceivable angle to mark and remove tiny lumps, swells etc that shouldn't be there - knots and natural movement in the stave taken into account of course!
That should allow you to brace it up almost immediately and see where you're at.
Tiller the whole bow at the same time - don't go for that middle moving, then tips stuff or whatever. Ash will kick you in the teeth at warbow weight if you try it!
Ideally, you want to tiller out to around 25", then do a good, deep heat treatment with the bow clamped flat or with slight reflex (heat gun, not butane torch!!). You want to get the wood so hot that although you're aiming the gun at the belly, the back is too hot to touch, but you don't really want any colour change. It'll need fiddling about with distance from gun to stave to get it right. Then tiller to full draw and do another heat treatment before sealing with epoxy or similar. Danish oil doesn't do anything against moisture, and traditional wax finishes are a pain to upkeep.
76" is a great length for a 31" draw by the way. You could go shorter if you come out too light, but as a starting point it'll work well.
