I've just caught up on your progress and it is looking really fantastic to me. I'd give the current rib depth a whirl. After all, taking off is easier than putting it back on, and with the long taper like that it is plausible that it will have the right sound. I don't know if there is a formula for rib depth. This fellow is building different shaped cornerless basses:
http://www.allbasse.com/
His design, which is sort of a rounded Savart shape, has a rather radical rib taper from bottom to top. I don't know what the measurements are, but I think it looked similar to what you currently have going.
I really like the way your barrel arching shaped up. It's pretty obvious the you have a good instinct as a builder.