First step was to measure up 12 inches from the bottom, then add another 1/2 inch so I knew exactly where the shelf would be.
Then, I cheated. I wanted 7" from the top of the first shelf, to the bottom of the next shelf. In order not to screw up adding or subtracting, I moved the tape measure to the 10" mark, at the top of the first shelf, then moved up 7". That meant 10" low end, 17" high end. 7 inches empty space. Perfect. Then, add another 1/2" for the shelf itself.
After one too many mistakes over the years, it's just easier this way.
