Add the right overage for any material and project type.
Running out of material mid-project is worse than ordering too much. A second order often means a different production lot with slightly different color or quality. The cost of overordering 10% is almost always less than the cost of mismatched materials or a second delivery fee.
After the project, store leftover materials properly — especially tile, flooring, and paint for future repairs. Label paint cans with the room name and date for easy touch-up identification.