I have a loop tied in my topping lift, which I just hook under the cleat at the mast. It's set for the "sail down" height of boom.
When I hoist sail, I just unhook it and the lift is set for the "sailing" height. Stays cleated at that setting. Quick, simple and no guessing.
Can't understand why, as someone suggested earlier, you would unhook the lift from the boom. It's a safety feature also, should a halyard break, or a shackle come undone, it keeps the boom from crashing down. and some booms are very heavy.