We recommend a 8 cm overhang per side. If you're sealing both the top and the bottom of the box, we recommend doubling that number.
For example, to seal a 30 cm box top, you'd need 45 cm. And if you're sealing the box from top to bottom, you'd need 90 cm of tape.
For the total measurement required, take that number and multiply it by the number of boxes needed.