How about this, 80% of the difference between the two teams, but there is a minimum team average and anything below that you get 0 and there is a maximum team average and anything over that you get 100%. So lets say the minimum is 775, if you have a 765 ave you lose those 10 pins and if the max is 1000 and you have 1015, you automatically give up the 15 pins from the get go and anything between 775 and 1000 is 80%.
I have used this type of system for a few years and watch the best teams bowl against the worst teams and it is an actual match when both teams bowl to there ability..