Start with the physical sheet
A4 paper is 210 × 297 mm. Check that the printer tray, print dialog, and downloaded file all say A4. If the printer quietly substitutes Letter, every row can drift even when the first label looks close.
Measure one label's width and height, the distance from the paper's left and top edges to the first label, and the gap between neighboring labels. Enter those values in the sheet calculator. The preview is a layout check, not proof that a particular printer will place ink exactly there.
Run a plain-paper test
Download the PDF and select Actual Size or 100% in the print dialog. Disable Fit to Page and similar scaling. Print on plain paper first. Hold that print behind the label stock against a light source, or measure the marks with a ruler. If the sheet is shifted but not scaled, enter an X/Y correction in the generator. If the marks grow progressively farther from the cuts, measure the pitch and confirm the paper size before changing offsets.
Use the printer alignment test to check the 100 mm ruler. Only load adhesive stock after both scale and position look right. Keep the first label sheet for comparison when you change printers, trays, or driver settings.