Don't be a slave to the 'book' measurements.
Mark the geometric centre, decide where you want the arrow and your hand to be relative to that,and work from there.
The arrow/hand position may be fine tuned or even turned upside down as you progress if the tillering dictates it.
You can't make a Yew longbow with a straight edge and a ruler, anyway with a longbow it's not as critical as a shorter flat style.
Sure you'll want to be checking thickness with your calipers and taking measurements, but never go by measurements alone, especially once it's roughed out.
Go by look and feel. I had a guy at the club saying his ash bow was developing a hinge as he was tillering it. Even unstrung you could feel it, I said shut your eyes and run your fingers down it, you can feel the thin spot, he was surpried by this, he'd not been using all his senses.
Sorry if that sounds pretentious
Del