Yep. 20% is very conservative and definetly safe.

Also don't confuse aerodynamic balancing (the topic here) with mass balancing ( done with weights ). They are two different items. But having said that it's good to add some weight as far forward on the surface as possible to move the CG of the surface forward of the pivot point for very high speed flying. With the pivot ahead of the MAC and the CG on or slightly ahead of the pivot I don't think flutter would ever happen shy of the transonic region.