Guess you're on about the flange around the stock where the bottom gudgeon is?
Of all the stocks I have, 2 are I14 stocks - these have about 0.5cm all around, but more at the front and back. I also have an aardvark stock which split up the back edge (including the flange). this has perhaps 0.75cm around the sides, but it tapers out slightly towards the edge.
Not at all an expert, but from this, I'd have guessed between 0.75 and 1cm all around, with plenty of material around the trailing edge to tie the back of the stock up.
What thickness of plate are you using?