Ok. This is what i have in mind. Do comment.
The min. he will get is 10% and the max is 40%.
Starting from 10%, he get an additional 1% for every $1m increase in company valuation during the funding.
So if the valuation is 30m or more, he get 10% + 30% = 40% If valuation is 20m, he get 10% + 20% = 30%
Fair?