Richard,
Sorry for last night's post. I love the sync-link, and it's the only way to fly. You've got the best possible part, so on to the question.
You've arrived at the deep end of the pool here. There's probably never been a dual carb set-up (or dual throttle-body setup, for that matter) that draws in perfect sync all the way up to WOT. There are too many variables-- how the throttle butterflies mount on the shafts, etc. Fortunately, it doesn't matter as much as you would think. What matters is what happens at idle up to about 1/4 open.
The good new is that it's pretty much impossible for a sync-link to pull the throttles in anything but a linear manner, as opposed to a hex-bar, which has so much slop as to be pretty much a guess. If I'm understanding the nature of your question, you're in perfect sync at idle, just not at a partial throttle opening.
The thing about idle is that you can (and should) be in sync without any throttle linkage at all. Adjustments to idle are made with the throttle-stop screws with the linkage out of play. Your linkage should always allow the throttles (both of them) to go back to the stops at idle. It sounds as if when you adjust the carbs to pull in sync at slight openings, the throttles don't return to the stops at idle. You can't have it work that way.
What you are looking for is very slight movement of the accelerator cable producing a linear movement on both carbs. I don't think I'd be looking at it (initially) with a snail, I'd probably be using a feeler gauge or something similar. I'd set up the linkage so that a tiny amount of movement on the accelerator cable (which will pull the 3/4 butterfly) has a corresponding pull on the 1/2 carb.
You could do this without the engine running, by holding the 3/4 throttle open with a feeler gauge between the adjustment screw and the stop, then seeing if the 1/2 carb is open the same amount (you'd check with a second feeler gauge). The only real adjustment you are going to be able to get is just off the throttle stops- making sure the throttles crack at just the same time. Once you've got it set up where they are on the stops at idle, and just cracking together, then I'd check with the snail.
Good luck.