Interesting thread. Tim and Brett have made very good points here.
Seems the easiest thing to do is use a handle with close spacing. I can't think of any downside to that, though I'd be interested to hear any thought as to why close spacing at the handle could, in itself, introduce a problem. "Pulling less line" with close handle spacing would be expected to amplify any linkage slop relative to handle movement, but I doubt that's an issue.
If the setup is typical, and there is a bellcrank to flap horn linkage, I'm wondering how to extend the horn, in practice. If it's a wire horn, removing it could be tough. An idea for lengthening the horn would be to "sister" on an extension, probably by bolting the extension to the existing horn. That would introduce a point of failure, and change the location of the pushrod, as well, which might make it not clear the wing. A different lengthening idea is a new horn, a platform type typical in R/C or for sheet elevators in U/C.
Tim's idea to measure how much line pull gets 30 degrees was interesting, and I measured my two planes. The one that tracks well in level flight measures 2-1/4". The one that hunts a bit is 3". The opposite of what I expected. But the rule of thumb is appreciated.
Peter