Good luck for today, I hope it goes well.
600 estradiol isn't that high. Some women don't feel better until it's around 800. But as long as you are over 400-ish, you have enough for preventative health reasons so you can also reduce if you think that would suit your body better. It can be a bit of a fine art all this, can't it? And sometimes it's not very predictable, like reducing one dosage sees it falling too low.
I think the problem is more that using utrogestan vaginally every other night wasn't giving you enough progesterone to oppose that much estrogen. I know Newson say it is okay to half the oral dose when using it vaginally, but the BMS say that this isn't okay and they advise using the same dose vaginally as orally. (I think it probably depends on the woman. It might be enough for many to use half the dose but clearly isn't enough for many too.) As the jury is still out on this one, personally I'd err on the side of using the same dose if I were to use it vaginally - but I don't have progesterone side effects.
Hope it goes well today!