@alda That's not a good way to quantify "dermal" estrogen. Its bioavailability is very different in transdermal gel form and in transdermal patch form, for one, and common application frequencies differ, too.
Patches are virtually universally rated not in the amount of the effective ingredient that they contain (although it's documented, and can be important for War On Drugs purposes when the ingredient is a controlled substance), but the amount of the ingredient that they release. The ones I have had experience with release either 50 or 100 microgrammes per day, and last for four days (nominally half a week, but weeks have an odd number of days in them). A patch might contain, say, a whopping eight milligrammes of estradiol in it, but most of it will remain in the matrix by the time the diffusion efficiency will have dropped to a pint that one should replace the patch.
A typical transdermal gel dosage is about a milligramme of estradiol, in this case, counted as the amount contained in the gel. It diffuses through the skin relatively fast, but not all of it reaches the blood.
(For order-of-magnitude context, a typical target level might be about 500 pmol/l. A mole of estradiol weighs about 270 grammes, and a woman might, on the bigger side, contain about five litres of blood, so all that target estradiol in her blood adds up to some 0.7 microgrammes; at a time, anyway.)
It turned out grumpier than I meant to.