for lapping stones i too like the granite tile with SiC also, low tech and inexpensive solution,
but nobody mentioned checking with decent straight edge both stone and lapping surface, not to mention checking true of straight razor
I rechecked some stones i recently lapped and they have a very slight convex towards center of stone, which i understand is the norm coming off of saw, guessing i have less than .5mm variation. much better than concave. some like the intentional convex but that is a whole different conversation.
worked on one near flat stone yesterday and it is not so easy getting it truly flat, inspite of having the right equipment