As stated in my last post I'm currently developing a new gambling website.

I got a few fine bankroll deals so I'm wondering what amount would be reasonable to make sure I can pay at any given time.

Anyone got ideas, suggestions or even experiences that would be helpful to share?

More than ....... how much you currently have.
Disregarding the numbers to make your worries be lessen, I suggest to limit the max bet but take note that it's not an assurance that you will not pay more. If your gambling site has a genuine provably fair option really expect a big winners.
The only thing I can say is, face all the possible risk.