Wizard,
When to sinew? What works for me is to floor tiller and then long-string, tiller the bow to the point which you can string it to brace height. At brace height, both limbs should be even in strength and have a good elliptical curve. I stop tillering when I've accomplished that. It is at this point, that I sinew back my bows.
Draw length depends on the what you've decided will be the length of the bow. 50" bow with a bendy handle can yield a 25" draw, with good tiller, un-backed with sinew. You could probably get 26-27 with sinew however, you'll have to be aware of the string angle at full draw. When you exceed the 90 degree angle your string loops can slip from your nocks. Setting back your tips is necessary to help insure against this catastrophe.
West Coast Paddle Bows if I recall, narrow at the handle then fade back out to mid-limb and fade back in at the tips. Since the handle narrows, you'll have to be thicker in the handle area than the rest of the bow. Tillering can be tricky.
With all of that being said, you mentioned that you've made two 70" long bows. Were they stiff handled? Might I suggest building a bendy handle design bow as practice prior to the west coast design with the sinew. Making a bendy handle can be tricky. Especially, a bendy handle with a handle narrower than the limbs. Processing sinew is a bunch of work. Would be a shame to go through the entire sinew exercise and the bow not turn out as well has you hoped. Please take my words constructively. You're venturing into
BowMaking-401
