When you cut it doesn't matter with BL. BL functions best as a heartwood but you probably won't have enough for a bow. Now, what happens in a living tree is that sapwood is eventually converted to heartwood as the tree grows. So if you must have a sapwood back get as close as you can to the heartwood. The sapwood will be stronger and denser. Look at your tree and decide. The closer the better. My fist shooter was made friom a BL sapling with a sapwood back. Jawge