A couple cycles my dr has given me an estrogen shot (E2) on cd10. From what I have read 8-13mm is considered good. Mine was 7mm, and I got bfp so there is no concrete cut off.. just some conditions are more desirable than others. Best of luck to you.
Ask your re about estrogen patches. (vivelle dots) I know on the one round of femara that I did my lining was only 8mm thick. It was not optimal but definitely at the lower end of what they like to see. The nurse did say it wasn't low enough to cancel the cycle though so we went ahead with everything. I am currently 27 weeks pregnant from that cycle. (I never did take the estrogen since I got pregnant but they said they would have put me on it the next cycle if it had failed.) I did take progesterone (100mg/twice a day) starting two days after my trigger shot...that can also help support the lining and aid in implantation.