A couple of ways round the problem.
1. Draw out the taper with straight lines on a piece of paper, floor, wall etc as a template. Measuer the thicness or width off the template at say 6" intervals and then transfer these onto the stave.
2. Clamp the stave to a straight bit of 2x2 and the you can mark directly on to it.
3. Don't over think it, just taper it by measuring every 6" and aim for about 1.5 - 2mm thickness change every 6". If it's right every 6" you can then blend in between each point.
Measurement is fine but try to get used to eye and feel, because that's what you'll need when you eventually pick up a wobbly stave to use.
Remember wood is a natural material and you can't simply build a bow by numbers.
I recently was asked to finish a Hickory backed Yew bow made by a cabinet maker who had since died. It was beautifully made but had never been on a tiller. It looked like a bow but was no where near right.
See this post on my blog.
http://bowyersdiary.blogspot.co.uk/2014/09/hickory-backed-yew.html It's all about using your eye, learning to see a good curve and spotting problems before they are obvious (it's too late then!)
I'm currently doing a boo backed Yew and it's about to go on the tiller, that's when the real work starts... keep an ey on my blog to see how it goes. It may be helpful to you.
Del