I don't think it is possible to give an equivalence like that - all the serum estradiol levels are averages from a populations of women being studied and actual readings are hugely variable. With gel particularly I think there is a huge variation between different women in levels and I read in a paper about pharmacokinetics of HRT (roughly - what happens to the hormones from the external source, how much of it gets into your body and is transported/metabolised, as time progresses) that one of methods - can't remember if patch or gel - had a ten-fold difference between the highest level ( of serum estradiol) and the lowest in the women studied. Also some women absorb the oestrogen better via one mode of delivery than another as we have seen on here. The best way is just to look at the approximate strength of the dose ie low medium or high. The standard starting dose for patches and gel or at least for post-menopausal women - ie a medium dose, is two pumps of gel and 50 mcg patch.
From reports on here some women achieve far higher levels of oestrogen on 1 pump of oestrogel than a 50 mcg patch but others barely absorb it at all. It's probably partly to do with application method ( for gel) as this can make a huge difference in how much is absorbed, or skin type or what you put on the skin. Body weight will have a bearing as well - all other things being equal, small thin women will achieve a higher concentration of estradiol from a given dose than tall fat women!
You oestrogen level should not fluctuate with a patch - as long as it stays on of course - and even if it comes completely unstuck levels only decline over 24 hours so you do have a bit of leeway.
According to the Australian Menopause Society in fact 50 mcg patch or 1 mg gel is medium dose (1 sachet of Sandrena) which is between 1 pump estrogel (0.75 mg) and the standard dose of 2 pumps (1.5 mg), so it sounds from this as though 1 pump is more like 37.5 mcg
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How long have you been using two pumps of gel and why did you reduce from 75 mcg to 50 mcg? Were your symptoms controlled with 75 mcg?
Hurdity x