I decided to cut my muslin in a straight 12 and see what happened. Please forgive the wobbly pinning at the front:
It feels like a good fit at the bum area. Any smaller and it wouldn't lie right.
There is a bit of ease at the hips, but I'm not sure how hip-hugging the pattern is supposed to be, so I don't know if it needs taking in or not.
The waist is too big, there's a fair bit of loose fabric here. But when I sit down the waist is tight so if I made it any smaller it might be too tight and cut me in half when sitting, not comfy during meal times!!.
Lastly there seems to be a bit of bagginess in the tummy area. Again not sure if it's best to leave his (my front pinning was very dodgy)