Looking at the pictures I can not see an issue with it. Ther are 3 points of contact bum, and the sides of both feet. As with regards the 'shouldering' of the rifle the great Terry Doe had the butt pad in between the shoulder and elbow to get the classic FT position.
If I was able to look at the rules (work has blocked the BFTA website
but allows this one) I think there is nothing about how low you can go as long as the rifle is not resting on the floor. As a slightly larger person
I would be nice to get down that low but the club grounds we use make it more difficult.