Every ad through the volume gate
At least three purchases in the window, or spend above twice your average order value. If an ad does not clear that, it is called too early and not a winner. That single filter prevents the most expensive mistake there is: scaling on coincidence.
Three windows at once
Three, seven and fourteen days side by side. A winner in two of three is a confirmed winner; only in the short window is a signal to follow, not a reason to move budget.
Creative rotation inside fixed limits
The weakest ad out, a reserve in, never below two live ads per ad set, and your best ad is never touched.
Pacing on the measured run rate
Not on the sum of your daily budgets, because you rarely spend those in full. The gap between the two is why monthly ceilings get broken.