I have three stones, all Shapton Glass: 220, 1000 and 6000. I use the 220 to flatten the 1000, and the 1000 to flatten the 6000. It seems to work well to achieve flat stones, but my concern is that it will shorten the life of the stones. Is my concern justified? If so, is it the "flatteting stone" (that is, the coarser grit stone) or the stone being flattened (the finer grit stone) that loses the most material?