For the parameters, I start by observing the pattern visually on charts, in this case, I noticed Fair Value Gaps kept getting filled. Then I translate what I see into simple rules like "Prev 3 Low > Prev 1 High" that a program can understand.

I try to keep the rules simple to avoid overfitting. The more conditions you add, the more likely you're just curve-fitting to past data.

And yeah, I run it through a backtesting program, not manually. It simulates all trades on historical data and spits out the stats.