Pitch is edge to edge
Horizontal pitch is the distance from the left edge of one label to the left edge of the next. Vertical pitch is the top-edge equivalent. For nonoverlapping rectangular labels, pitch equals label size plus the gap between labels on that axis. The two pitches may differ.
Measure across several intervals instead of one. For example, if five column intervals cover 315 mm, horizontal pitch is 315 ÷ 5 = 63 mm. This reduces the effect of a small ruler reading error. Use the actual cut edge rather than the printed design, which may have its own inset or bleed.
Record the whole layout
Also measure the sheet width and height, the first label's left and top offsets, the label width and height, and the number of columns and rows. A stock template should retain these values separately. The template finder only publishes layouts with an identified source; an unsupported width alone is insufficient to recreate a sheet.
When building a custom sheet in the generator, enter gap as pitch minus label size. If that difference is negative, recheck your measurements rather than entering a negative gap. Confirm the last label still ends within the paper.