Here's a website that has an article that I wrote on boost tabs and servo tabs for M.A.N a few years ago.
http://www.geocities.com/roger_forgues/Boost-tabs.html
When initially setting up the travel on boost tabs, it is best to start with the boost tab moving about half as far, angularly, as the control surface, then gradually increase it. If you overdo the boost, you end up with a control surface that constantly tries to deflect, rather than to weathervane until the servo attempts to force it off center. The result is a control that may refuse to trim - the control surface tends to flip one way or the other within the linkage slop that is always present. On one of my early rudder boost tab experiments, the rudder disconnected from its control linkage when I stupidly forgot to replace the servo output arm screw. The rudder went hard over in flight, making things get interesting fast. That particular model would only fly in knife edge with its rudder at full deflection, so I had to "land" the model in knife edge, but very little damage resulted since that fully aerobatic big bipe could fly much slower in knife edge than its normal stall speed.