Great point. The drawer width was taken on a story stick, not numerically. I first made I side spacer blocks and then made a sandwich on the old shelf with a spacer block and a 1" block (each slide takes up 1/2") and then marked off the gap on a stick.

Observant folks might notice that I made the drawers sideways. Normally the rabbeted piece is on the side to use the mechanical advantage of the joint. But then measuring the length of the front/back pieces also involves the joint. I wanted make this task as easy as possible so I used the full length, rabbeted pieces on the front & back.