I don't actually measure the thickness, but then after you've turned hundreds of them you don't need to. The general rule of thumb is that you leave a wall of 10% of the overall diameter. The problem with this ratio is you are restricted to producing set thickenss bowls. When you gain some experience of various woods, you often find you can leave them thicker than 10%, which leaves some lattitude for the eventual shape, wall thickenss Etc., etc.
I usually just coat the endgrain areas with PVA and stack them somewhere dry and out of draughts. The success rate is very good, and I don't lose many at all.