Find where the print begins again
A print repeat is the distance from one point in a design to the next matching point. For a large motif, moving one panel until the design lines up can use noticeable extra length. Lay out the actual fabric and find the repeat in the direction you'll need to move the panel.
Start with the panel sizes
Use the backing calculator to find the rectangle, panel count and panel lengths first. Those figures include the overhang and seam allowance you enter. They don't include extra fabric for moving a print into position. Treat the result as the starting panel plan, then work out the matching allowance on the fabric.
Keep a separate note of the matching allowance
Write down how much extra length each panel needs for the match you want. Add that to the material you plan to buy before rounding to the shop increment. The backing form doesn't have a print-repeat field, so keep this adjustment with your project notes rather than increasing the seam allowance to hide it.
Check the match before cutting every panel
Spread out enough fabric to see both the intended seam and the repeated motif. A match that looks right at one point may not suit the whole quilt back. QuiltMargins doesn't read the print or arrange matching panels automatically; that choice needs the actual fabric in front of you.