I do not know the brand or composition of the ones your using, but here are a few links that might help give you some guidlines.
Spirit of '76 Gun Page
E. Arthur Brown Company
While doing your testing you might try something a tad heavier, and you might get a little better accuracy. Being that the 55's normally work well with a slower twist, something in the 62 - 70gr. range, might work out even better in a 1-10 twist. Just something to consider.