There is no restriction Andy, but it becomes problematic because it makes the rifle more tippy/flippy as the CofG goes further up away from the support/pivot point... but sometimes it's necessary to a certain degree. The other option is keep the fore-end shallow and raise the scope... sometimes you have to do both.
You will tend to find the more flexible shooters can use a shallower stock than the less flexible... certainly there seems to be a correlation between the ability to get the knee into the chest with ease and the distance between scope and bottom of fore-end.
Kneeling tends to favour a deeper fore-end, sitting as well, but standing tends to be better with less... especially when combined with butt movement up and down.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep