It looks to me that you are picking up on this very fast Abbas. I had problems with the wandering ruler too, although I used a straight edged piece of wood to guide my veneer saw. I found that It helps a lot if you can clamp down one end of the ruler/guide when possible and use your free hand to hold down the other end while you cut. I think a piece of wood is better than a ruler for 3 reasons: (1) the wood can be a lot thicker than a ruler and therefore easier to keep your saw tight to the edge of the guide at 90 deg. (2) A wooden guide can be much wider and therefore make it easier to keep your fingers further away from the cutting edge for safety. (3) Wood clamps better than thin steel.
Mike, an American living in Norway