Having just spent some more time messing with the sitting stance I think the majority of the 'wobble' is coming from a pathetic lack of 'Core strength'.
Using the front hand on knee and right elbow resting on inside of right thigh method, I'm having to lean back to get my head/eye into a decent position. Lower back muscles and tummy muscles are working hard to hold the position. If I try and sit more balanced and upright it would mean I would have to have a silly deep fore end or silly high mounts. They are both quite deep as it is. I suppose Core strength can be worked on and will improve the more one shoots in that position.
I hope I'm not opening up a can of worms ... seeing as there are no rules in FT
... but if I place a small cushion behind my bum/lower back, and lean into that, the position becomes steadier. So can I just push back into a FT cushion so the cushion swells up at the back to almost form a back rested seat? I know the rule says something like max depth of 4 inches or something, but is that laid out flat with two boards over it ... and you can force the fillings higher in one area?