I am giving cycling a break for a week or so while my back recovers and the pain I have been experiencing disappears. It is starting to feel better and I think it was because I hadn't rotated/flipped my mattress in a few weeks and this was causing it to sag more and create problems when I sleep. It is a firm mattress (or supposed to be) but it's never really felt very firm to me, more like a medium than anything. I am hoping to get a new one some time in the future.
Anyway, I measured the distance between my bike seat and the pedals when furthest away and it's currently about 86cm (34 inches) and I think my inside leg is roughly 74cm (29 inches) so perhaps it's a little too high. Using the 109% method then I should be looking at a pedal-to-seat height of roughly 80cm, when I lower the seat to the next notch down then it is still over this amount and the next notch after that and it is slightly below at 79cm.
As I said, I will wait a week or so and then have a test with some lower seat heights and see how I get on.