I cannot think of a better model than what was used in the system.
If you know better, let me know pls.
No, I'm not asking for millions. It is up to the customer who want to trade his account with this system. The bigger his account is the better it is regarding absolute profits, smooth equity curve, small drawdown (aka small risk) etc.
An account of size 600k is IMO an ideal size, for bigger accounts more titles would need to be traded, and this could mean adding some more exchanges.
I'm asking only for 15% to 20% (negotiable) of the profits the system makes.
What are the parameters of your Geometric Brownian motion model?
What formula are you using to calculate the option price?