Blood levels are of limited value when using gel as it peaks rapidly in the plasma and then drops off equally rapidly.
Minimum levels considered adequate for maintaining bone health in the majority of women are 250-300pmol/L. A study by Sarrell et al in the 1990s outlined approximately 184pmol/L (50pg/mL) as being the threshold to avoid urogenital atrophy - therefore the presence of GSM can be considered a harbinger of osteoporosis. Note these levels are minimal not optimal, and thresholds for other outcomes are not well defined.
Personally I will be aiming to maintain above 400pmol/L which has been shown to be bone positive in the majority as opposed to merely slowing the rate of loss relative to placebo. This also approximates the average level across a natural menstrual cycle outwith ovulation, and is also above the early follicular phase i.e. during menstruation, when most women don't feel their best.
It is possible that indirect markers can be used to assess the adequacy of dosing whilst on gel, such as FSH or SHBG, however these have not been validated for clinical use and do not form part of standard practice (however nor does checking any blood tests at all).
Additionally you could follow symptoms (obviously highly subjective and not universal, plus the fact that just a whiff of estrogen can dramatically reduce vasomotor symptoms but not alter the health trajectory of undertreated women), lipid profile, serial DEXA scans and serial coronary artery calcium scoring, however these have practical limitations and you don't necessarily want to wait 5 years between DEXAs for example to find out you are losing bone hand over fist.
