Something like the success factor?
Fs = (Good Photos taken) / ((Missed shots)+(decided not to)+(should have brought the other lens)+(not so good on print)+(Ruined the neg)^2)
Any ruined neg / light leak / bad overexposure is squared for the extra aggravation this causes...
So far, my Fs is around 0.05 - which is good. It means I'm happy with at least 1 photo!