Uterine lining

I’m getting ready for my first frozen transfer and they triggered me yesterday to have the transfer done next Wednesday. However, my uterine lining was 6.97mm. My clinic wants your lining to be at least 7, but I’ve seen others say 8 or 9. I know they wouldn’t proceed if they weren’t confident, but we only have one male embryo that were planning to use (we have 3 female and plan to do a female next). There’s a lot riding on this one so I just don’t want to risk anything if it doesn’t work. I guess my question is what do you know about uterine lining, should I be worried and wait or should I listen to them?