@ryanc with the regular blood tests from the endocrinologist or whatever doc does them for you (most people get one every (half) year)
doing it the month before should be enough to stabilize levels to whatever (small) influence it has.
i would guess the difference is just in the 10pg/ml range for estrogen gels. not much but still interesting to take a closer look at (since i could not find any studies on it)