We use the following:

1. A calibrated, high-precision granite slab
2. A calibrated square with an accuracy of GG 0
3. A feeler gauge

This setup is the same as recommended and used by leading plane manufacturers. It is important that the sole of the plane itself cannot serve as a reference surface, but only as a neutral base, a kind of auxiliary reference point, which must be very precise. Particular attention must be paid to ensuring the surface is clean, as this can affect the result. A calibrated square (not a hair square) is then placed on the granite slab and brought close to the sides. If no gap is visible, the accuracy is perfect. If a gap is visible, a feeler gauge must be used at the open end of the gap. If a 0.002 inch (0.05 mm) feeler gauge does not fit between the two, the accuracy is within tolerance for manufacturers such as Veritas. If higher accuracy is desired—though far from practical for woodworking—it must be achieved by the user using standard lapping techniques. With Juuma planes, a 0.15 mm feeler gauge should not fit between the plane and the surface.

To measure the flatness of the sole, place the plane on the granite slab and attempt to insert a feeler gauge at various points. The tolerances are as described above (this applies to Veritas planes; a higher tolerance must be accepted for less expensive models).