I used a wood-carving gouge at first, then switched to a curved scraper. I am using the mass calculator to keep an eye on the weight of the bow, but mostly it is by feeling limb thickness between my finger and thumb - weak spots still show as twist or hinge on the tiller in the same way as a flat bow - I have not gone deep enough in the centre, and I need to take the hollow more out toward the edges - the biggest draw back is that it takes a lot longer than building a flat bow, but, I think it reduces the chance of getting crysals (compression fractures) and it does look good