I can't provide you with any numbers. If you aren't overclocking, though, and the card is within warranty period, I'd get an RMA from Sapphire. You shouldn't be having that problem.
Beyond that, I'd just start at the default for a given BIOS, and lower by ~25MHz increments until the artifacts go away.
You should also find a reproducible game scene you can check performance with, and try lower and higher GPU clocks. It's quite possible for an overclock (any NITRO+ card is overclocked out of the box) to not provide a performance benefit, due to it introducing errors that must be retried at various levels.
If you really want a number to target, look at the out-of-the-box numbers for that card, and the stock 5700 XT numbers. Start with the latter, and work you way up to the former until you run into either artifacts on the screen or performance degradation.
Unfortunately, there's no solution to your problem that doesn't require some kind of time investment on your part.
