Ken makes a good point about the square. What do your eyes tell you?
Have you tried these on a shooting board and checked the result? Shoot two short pieces cut from a shop bought piece of wood, so you can use the same faces against the base and fence of the shooting board, then lay them down on the bench and bring the shot ends together. Check to see if there are any gaps top and bottom. Or you could lay two pieces of man made board, flat on top of each other, and slide the top one away so that there is room for your test shot piece. Bring it in to contact with the top piece and check for a gap.
The first method will show twice the error that the second will, so may be easier to see, but it does rely on you having a good flat reference surface to lay them on. The second method uses the lower board as a reference surface for both the upper board and the test piece.
Colin, Czech Rep.