It should be pointed out that not all manufactured plane irons and chisels are uniformly flat along the full length of their backs. As a result there are occasions where some lifting is required to insure that that the forward 1/2 to 1 inch is in direct contact with the surface of the stone. That's a primary reason why its deemed good practice when working these backs, to apply some downward pressure with your forward fingers close too the cutting edge. Whether 1 chooses to use the Ruler Trick or not, what's has come out of this discussion are the real benefits in keeping the top surface of your honing stones within a tight tolerance of flatness. That in itself has some relevance to Derek's inquiry within another post, as to why you would bother keeping your stones flat. Out of flat stones may be seen as acceptable for working the bevel side of your plane iron and chisels, but that's clearly not the case when working the backs.