Zone testing is the classical method to divide the mirror into smaller zones and measure the RoC of each zone to calculate how far the mirror curve is from an ideal parabolic shape.
Usually, you will make a 3, 5, 7, or more zone mask and calculate the difference of RoC between each zone by using the micrometer of the Foucault tester. Then these numbers will be fed into a data reduction application (Figure XP, Foucault Analyzer, etc.) which will automatically calculate and return the PV rating, Strehl ratio, and other details to gauge the quality of your mirror.
The below image is to show you what zone testing looks like and the movement of shadows and graying both left and right zone simultaneously while taking zone readings. The last one or two zones are always tough to interpret.
The above pics are taken during the correction phase of my 8-inch F/6 mirror.
Astro Khan Telescope Making 