My rule of thumb is simple. If a leaf is covered with trichs, it stays. Otherwise anything not covered with frosting goes. Some strains have very little on the leaves, so I trim them CLOSE. Other strains have leaves that are absolutely covered with frosting and those I leave on the bud. Its not complicated, just tedious.
I save all my trimmings for canna butter so if I trim a little close, So I don't really loose anything.