My suggestion for flattening a oil stone is to take a piece of steel plate about a foot square by 3/4" to 1" thick. If torch cut, grind off TC edges so no positive metal sticking up. Flood it with mineral spirits and start honing the plate. Consistently rotating your stone about 90 degrees per stroke. Keep adding mineral spirits and stroke the stone for about five minutes. By now the stone should be flat. If it still shows low spots that did not get touched, keep honing, you will eventually get it flat. Plus, you will wind up with a reasonable flat piece of steel plate! Surface plate quality, maybe.