Step 2 · Establish the Finished Floor Line
All vertical plumbing measurements are based on the height of the finished floor, not the subfloor. Determine the total thickness of your flooring materials (e.g., 1/2-inch backer board + 1/2-inch tile = 1 inch total). Measure up this distance from the subfloor and use your level to draw a crisp, perfectly horizontal line across all the wall studs where the vanity will be located. This line represents your finished floor and is the zero point for all subsequent vertical measurements.