You're missing the point. You have no idea how many turnovers you need, so GPM is of no help. Your eyes are your guide. If the pool is stupid clear, chances are you can lower your filter time. If it's lost some of it sparkle, then you need more filtering. The need will change drastically over the season. Alot, some, next to none, some, alot. Or if you're like most people by us they open late and close early to miss the 'alot' parts..if I want to run my filter to get X amount of turnover daily how can I get there by not knowing gpm..
Again, with or without the cover, most people are by default covered filtering with their SWG runtime.
I do this without knowing my GPM or how much I'm turning over.

My VS pump runs 24/7 at 1500 rpms for the cost of 2.3(?) hours on max rpm. I could go considerably lower but we like the water moving a bit more so I splurge all the way to 220W. Technically 35W / 700rpms activates my SWG, skims and filters.If I'm doing your method why get a vs pump just use a single speed run for like 4 hours a day..