I bought an 18 inch stainless ruler at Home Depot it is a good ruler made from good steel.
One day I wanted to measure off a 90 degree edge and to my dismay I realized the folks didn't put the starting line at the edge.
This makes it worthless to me.
So today I am meticulously fixing it, I ground the end down to so far and am stoning it to the last amount and keeping it square at the same time.
This takes awhile as the steel is rock hard.
You cant do this with any old cheap stone.
After fooling with it for a bit it is square I checked it with the height gauge and it is reading within 0.002 of accuracy when the tip falls in the slot checking several times.
Not bad for a ruler calibrated by hand and an India stone.
Here is a sketch showing how I kept it square using the side of the counter as a stoning guide.
Worth
ruler.jpg