Calculate the net requirement
Start with the actual surface area, volume, cut length, or number of pieces.
Name the source of waste
Kerf, trimming, defects, mixing loss, spillage, overlap, and minimum package size are different sources of waste.
Add waste after the base calculation
A percentage allowance should be visible as a separate step, not buried inside an unexplained number.
Buy in real package sizes
The final purchase must account for board lengths, sheet sizes, container sizes, and the possibility that matching material cannot be replaced later.
The calculator is one part of the job
Use the result to support a decision, then compare it with the drawing, material, tool, product instructions, and actual conditions.