Tall Paul is right - a boost tab is a completely different animal from a balance tab. Balance tabs are simply a portion of the control surface that is ahead of the hinge line. I experimented a lot with rudder balance tabs before largely abandoning them in favor of boost tabs. About 10% of the area is more than adequate for most applications, but be sure to make the linkage adjustable, and start with about 50% of the angular movement of the control surface, then gradually increase the tab travel until you get the control response you want.

Balance tabs can reduce servo torque requirement by about 85%, after which non-linearities rear their ugly heads, resulting in "funny" feel and response, particularly on rudders. I equipped a rather large bipe with aileron balance tabs, and one servo produced at least twice the roll rate that two servos had previously produced - a vast improvement, and it saved weight and battery current too.