In short - The proper dose to maintain an E level that sustains feminization is critical to a smoother and successful transition. So unless you have a health risk, you need to have the estrogen level at least near 200. My doctor is a Transgender Specialist and she said at low levels a girl will maintain the status quo, AND NOT REALLY IMPROVE. She understands that her patients can sense low estrogen levels, and she immediately brought me up to 212 after my former Endo kept me between 100 and 140. The estrogen levels of a 62 year old woman were doing me no good and had stalled me out. Getting above 200 made a big positive change in the way I felt, and it kick-started the body changes again.
Anyway, levels below 100 may even reverse some skin tone and body hair to a slightly more MALE pattern. And that's not good for your transition or mental state, and how a girl feels inside and about her body is far more important than some doctors think. (They can be so damn stupid and overly cautious at times.)
I'm doing spironolactone pills, with self injections of depo estradiol every week. The normal two week injection schedule had a dramatic roller coaster effect, so I was switched to weekly injections at half the two week dose. It's cheap and effective for most people ... unless you have good insurance or get the super inexpensive Walmart prescription pills.