I experience the same with low blood pressure. Originally I was taking 40mg morn and 40mg night (80mgs daily that the Dr recommended). I found the side effects too high so after a while started taking just the 40mg of a morn.
Realised after missing over a weeks worth (accidently ran out), how much i felt different and so realised i was feeling light headed, dizzy and nauseas from the tablets. Tried the 40mg of a night time but was having the weirdest dreams and was unable to sleep well so now trying to halve the tablets (so 20mg morn and 20mgs night) and so far so good. I did tell my Dr i had reduced to 40mg for the day and he said that was fine. He also told me i could stop them if i reduced down slowly (didnt just stop cold turkey) but i havent been game to stop.
I know if i dont take them for a week or more, i start to get dull headaches every day and based on my past, this normally eventuates into a full blown migraine. So im sticking with the 20mg morn and night unless migraines start then i'll go back to 40mg of a morn only as that seemed to work for a long while.
I guess its all trial and error and its hard to find that balance between BP and migraines. I did try some other migraine preventative before but it didnt do much so i think the propanolol is a god send really.