I like to set the travel of boost tabs a bit on the low side of what I think will be needed, for initial testing, then gradually increase travel until I get the response that I want. If you use excessive travel, the control surface will try to move off center on its own, forcing the servo to center it, rather than deflect it. This is evidenced by a control that just refuses to trim, since the surface tends to flip and flop back and forth within the linkage slop, unless the boost tab is initially deflected a bit to act as a trim tab, something that boost tabs do very well, incidentally.

I came to grief with an over-boosted rudder that deflected to full travel on its own, after I neglected to properly secure the linkage after adjusting. Had to land the bird in knife edge.

Have done a lot of experimenting with boost tabs and servo tabs, and wrote that article that Iron Eagle posted - it may still be available in Air Age's "How To's, Vol. 2".