I am not on HRT, although might be in the future.
My assumption is that our own hormones are fluctuating which gives symptoms and HRT raises the levels to prevent this happening. In my mind it would be impossible to know when the fluctuations were happening and attempt to add in the HRT at these points - which is sort of what it sounds like you are thinking might work. The HRT needs to be at a level that lessens these peaks and both estrogen and progesterone are needed.
I also wanted to say that I have tried the route of understanding which hormone is causing my symptoms or which one is high or low and for me personally, I am fairly sure that progesterone is low. Please don't assume a bleed means you have adequate progesterone, the bleed can be with high estrogen and lower progesterone or it can be from just high estrogen (breakthrough bleed). Lower progesterone can cause PMS in the second half of the cycle. The issue is more about the balance between the two I believe.
As much as I wish it were possible to work out things the way we need to feel 'ourselves' again for the majority of the time, I don't think it's possible. What is possible is to go back to your GP and explain what is happening and get advise.
Maybe some more experienced ladies can advise you more specifically about dosage etc but personally I think you should stick to what was prescribed and seek advise to adjust it x