Have them on the ailerons on a Corby Starlett. Rule of thumb is 10% of the area. Had three holes in the boost tab horn. I kept moving in on the holes till it felt like you put the next size bigger, faster servos in it. When it does that, don't go any further as it will overdrive the surface and will not stop oscillating. Ended up, with full deflexion (sp) the tab was about parallel with the slipstream. Hope this helps.