Aha! – that’s where the tweaking comes in – you’re right, you may have to pause at about 90-95% fully tight on the reefing line (boom is now up in the air, i.e. scandalised), then drop the mainsail to hook the cringle onto the horn (boom is now slightly below normal height, but not much), then completely tighten the reefing line. Depending on the position of the reefing horn I admit that there is scope for the boom to drop fairly low, although not enough to persuade me to fit a topping lift (yet). But if it works for you (and especially if you’re used to having one available, because of your previous sailing experience) why not? Go for it!