From what I’ve learned, estradiol levels for bone protection are usually discussed in ranges, not strict cutoffs, because every woman absorbs transdermal estrogen differently.
For maintaining bone density, many menopause specialists note that serum estradiol levels around 40–60 pg/mL are often sufficient for most women. For women trying to increase bone density, especially in early osteoporosis, levels closer to 60–80 pg/mL (sometimes slightly higher) are associated with more bone-building activity. These numbers are based on clinical observations rather than official guidelines, so they’re best viewed as general targets.
I currently use a 0.05 estradiol patch, and my most recent estradiol level was 60, which falls into the “maintenance” range. Patch absorption varies a lot between individuals, so the same dose can lead to different blood levels in different people.
There is also emerging evidence that testosterone, at physiologic female doses, may provide additional support for bone density, muscle strength, and fall prevention when used together with estradiol. I'm using a testosterone gel, 5 mg a day.
Many clinicians do combine estradiol with osteoporosis medications (like bisphosphonates or others) when someone is in the early stages of osteoporosis. Estrogen helps stabilize or rebuild bone, while osteoporosis drugs work through different mechanisms, so the combination can sometimes be more effective.
I think, if someone has osteopenia or early osteoporosis, a higher-dose patch may help raise estradiol levels into the bone-building range, but it’s very individual.