Interesting, TBH I felt like an idiot because the tooltip says -25 one turn and on instability it says -8 decreasing 1 per turn (DERP - its in the tip) - I just ran a test on hard difficulty and did discover a cool thing

1 - attack 1 city and occupy
-25 one per turn
then -8 that reduces once per turn

HOWEVER
I went on to take another city in the province while i was still at -4 on city one
- 25 one turn
and - 8 that reduces once per turn

so the -8 was capped at 8 i.e. if I took 2 cities at once I would get -8 in each rather than -16
result