Mock my methods as you will!

When measuring intend not to trust myself. So I use something to make the marks easier to see. Then use the old MK1 eyeball.

Left hand row. Is good.
Right hand row..... Was not. 1 and 2 worked. But because the wall moved where 3 is. It was 20-25mm out (nearly 1 inch or 1/430th of a London bus)

Found a straight edge to make sure as well. But MK1 eyeball FTW. Am glad it was hammered into me during my apprenticeship.

Other side was all within 2mm >.>