itguy,
My setup for the topping lift is near identical. I have the stopper you originally posted as well. The wheel "tightener" thing on it was gone on it when I got the boat, but Rudy at D&R has replacements for a few bucks
I would get one from him if you have not already, it does help.
http://www.drmarine.com/proddetail.asp?prod=DR20%2D90What I did is...
On the trailer, drop the stopper in the lower part of the mast beneath the wide opening for the Main sail. Hoist the main all the way up as far as it will go. Pull the Cunningham down as far as it will go. Now you have your sail pretty much stretched as much as possible. It is essentially filling as much of the mast track as it can. Now bring the stopper up about an inch or so below where boom is, and tighten it up good... otherwise it will get itself loose again from the vibration of the trailer when you travel which will mean either you have to reset, or at worst replace (if it falls off when on the road). At this point, I have not ever touched the thing again. That position just works for me. I am 6ft tall, and on my boat I think that a person a few inches taller would still be fine with the height of the boom, sitting in the front of the cockpit of the boat.
On the topping lift... The PO of my boat mounted a relatively large block for the application halfway between the shroud tangs and the top of the mast (actually this was probably for a spinnaker??, but he turned it into a topping lift). Doesn't really work that well, and have traditionally bastardized it in favor of just dropping the boom and sail in the boat. About a week or so ago, I got a micro swivel block and put it on an eyestrap (spare parts) at the top of the mast. It uses a carabiner (from the old TL) attached to some 50ft FSE Robline 3/16 that I got on sale, and ties off on a cleat on the side of the lower part of the mast. The system did not make sense to change because it was mostly already there, just needed the block to be at the top of the mast. With that said, again on the trailer, with the main sail up... slack it off so the main is not impeded from filling with wind. Then tie it off. When I drop the main sail, the boom drops a few inches, but there is still pretty good head room without having to adjust anything. However, if you need to adjust, with your setup you can crank the boom up if you like. You will just have to re-adjust the TL slack again next time you raise the main.
And just for old times sake, if I want to drop the boom into the cockpit, I went ahead and let it all the way down before cutting off the extra TL line so that it would still be able to tie off and not flapping around if I wanted the boom was all the way down.
As a bonus... well maybe... You mentioned making your jib downhaul. This was another recent project of mine that I screwed up... but made a discovery in the process. Well, after talking to Tim Webb, I know I am not the first one to discover it, lol. So, I put a flip-flop micro block up by the stem head and a horn cleat by the chainplates (once again courtesy of my good friend Tim's advice), and for whatever reason (maybe beer?) I cut the line for it to short. So, even though I screwed up, I still wanted to play around with the new jib downhaul, so I was looking for a place to tie the aft end of it off. Well, apparently I cut it just short enough to come back to the mast. I put a piece of the line I trimmed away from the TL around the mast just above the horn cleat (for Cunningham) that clears all the stuff my jib sheet like to get snagged on. Well, with that in place, the jib sheet crosses every time due to the lift that occurs, bringing the jib sheet up and over all the "snagables". The only downfall is that there is a trip hazard up on the foredeck, but with the way the boat is rigged now, I should not have to go up there unless setting up or tearing down the boat.