The grooves I cut with a steel electrical conduit,(this is very old material, would never pass inspection), 15 mm diameter resting on a board on the rim of the trough, all very non-technical, you understand. I think this pipe form is ideal even though it wears quickly. Since it's a pipe there is always a sharp edge to be found by rotating it where something solid would sooner become rounded over. I begin near one edge of the stone, the pipe only making contact at the high points. When the groove is continuous it means the low spot has been reached and I move on to the next groove at the stone's opposite edge and so on and so on. With this method the surface may or may not come out straight but this doesn't bother me since I am grinding blades free-hand without a fixed guide. The only concern I have, that the grinding surface be flat and even.
I always work the stone with water because in the case of grinding I think it is most effective, in the case of trueing I have no dust coming off.