It's hard to say, you need FSH - not estrogen. Estrogen levels can vary even in a pre-menopausal woman from 50 to 800+ over the course of a month. Which is why it's not reliable to look at estrogen levels.
The HRT has to add in a stable amount for you as an individual, if you keep the same level of patch or whatever, I'd have thought. That's what it's designed to do.
Were you always a 75 patch, across all those tests? If not, it's a bit impossible to figure anything out...