£80m pound fund. Assuming it does 2x in 10 years, with 0% hurdle rate, 20% of carry will be, 16m£? divide it by 20 entrepreneurs and you get £800k. Not bad...
One thing going for founders' mutuals is diversification. 100, 500 companies instead of 20 companies should work better for founders, as described here: http://reactionwheel.net/2015/06/power-laws-in-venture.html