I do not know the answer to your question, but I see where you are coming from.
FSH levels vary widely before, during and after the menopause in response to varying levels of Ovarian hormones.
If your Ovarian hormones fall, then the FSH should rise to urge the ovaries to produce more hormones and when the ovarian hormones then surge into the blood stream, the FSH should fall.
Hence I would have thought that if you were taking HRT for menopausal symptoms due to low ovarian hormone output, the FSH should remain normal.
I suppose one could argue that if you were on HRT and your FSH was low it would be an indication to lower your dose of HRT.
Sorry that I cannot be more specific.