Well, in fact, everybody has like 50% chance to be right 😄 Because the price moves up or moves down, if the EA can add something extra, this chance can be increased and if you always has the same sized positions, I think this can result in more linear equity curve.
I usually like to calculate my own risk, maybe this will change in time as I'm gathering more and more experience, but currently I like fixed lot options 😄
Anyways, I usually take the smallest possible positions for tests, my broker allows 0.01 on my micro account, if the EA proves profitable, I can always change that, I'm in no hurry 😄
edit: 50% chance, theoretically :)
Sooner or later, but it will work.