We rebuild the page closest to your sale and run it against your current one on your own live traffic. The scoreboard is revenue per visitor, not a dashboard metric.
Usually the product page, because that is where the money is decided. One page done well, not a list of opinions spread across the store.
Old page and new page live at the same time on your traffic, so seasonality and ad spend hit both equally. The result decides.
Two or three tests a month after the rebuild, each one measured the same way. Fewer opinions, more tested changes.
Conversion rate alone hides a page that converts more people at a lower order value, so every change is judged on revenue per visitor from your own store.