My old boat (26 feet) had a topping lift that just ran from the top of the mast to the end of the boom, where it attached via a cleat. No adjustment offered. All I did was tie a loop into the line a couple feet from the boom and tied the end of the line to the cleat.
So, when I wanted the boom up, I just pushed the boom up and slipped the loop over cleat. When I wanted the topping lift loose I just pushed up on the boom a little and pulled the loop off the cleat. The topping lift went slack, but remained tied to the cleat.
It was just a matter of fiddling for a few minutes with where to position the loop, and where to attach to the cleat, but once set, required no further fussing.