Clive
Depends what average you want? or what is requested for a competition.
Probably the most common is from 100 so yours would be 85.6 on that basis, but competitions will normally specify the basis,
i.e the British Championships is from 600 (sixty shot match)and is specified as the best 5 of your last 6 sixty shot shoots. So if you shot the same over those your average would be 513.6
Have Fun and Good shooting
Walther KK500 Alutec expert special - Barnard .223 "wilde" in a Walther KK500 Alutec stock, mmm...tasty!! - Keppeler 6 mmBR with Walther grip and wood! I may be a Walther-phile?