I think you're over thinking it:
every users sets their percentage on a scale from 0-100% (someone may want to go crazy and make it 100%, literally gambling).
every users sets their percentage on a scale from 0-100% (someone may want to go crazy and make it 100%, literally gambling).
Then someone with a huge bankroll could manipulate the maxbet in such a way that the risk of ruin for the site is huge i guess?
No, no. Let's say investor A sets his risk to 100% and B sets his to 1%.
Now the whale comes along and bets maximum and wins. A will lose all his money and B will lose 1%. Just as it should be.
Sounds good to me.