I've had good luck in making my boost tabs mechanical linkage such that the boost tab stayed parallel with the fixed surface no matter what degree of throw was on the movable surface. I did find out that greater than 10% area did cause a problem (would hunt for neutral and tend to gallop when the elevator boost tab area was greater than 10% of total elevator area). I never made any actual measurements but effectiveness seemed to be exponentially proportional to air speed; i.e. more effective at higher speeds.