There are various different opinions about what the minimum level is. Newson doctors told me about 250-270. But other sources say slightly higher. And then - it is dose dependent. Meaning, if you go higher, you improve your bone density further and things are even better. But I think all would agree below 250 is too low.
The other thing to say is that during peri a lot of symptoms are due to fluctuations. The ovaries are like an engine sputtering which is running out of fuel - a huge spurt of estrogen and then nothing, and so on. With no pattern to it and it can vary from hour to hour. It is these dips and spurts which cause a lot of the symptoms. To buffer out the dips and reduce the fluctuations, it can take quite a high level of estrogen (like 400-600pmol) from HRT during peri.