Mr. James Mulherin, Cheif Optician and CEO of Torus Technologies thinks the P-V Wavefront Error measurement is too harsh on a mirror.  I can see why he would be more amenable to a less stringent test.   Mr. Mulherin quotes Mr. Daniel Malacara's "Optical Shop Testing" in which Mr. Malacara says:
"The Peak to Valley Error must be regarded with some skepticism because it is calculated from the worst two interferometric data points out of possible thousands. It might make the system under test appear worse than it really is." ( The underlining is mine. )
To me, this sounds like an industry closing ranks to defend an inferior product standard.  However, Mr. Mulherin does have some valid points to make, so I shall continue to quote him with occasional editing remarks of my own.  He goes on to say: "A mirror with a true P-V Wavefront Error of l/4 as verified by interferometry, will in any case meet or exceed the RMS Wavefront and Strehl Ratio criterion described below." As a programmer I can visualize the required 'C' code needed for the software that performs the statistic Mr. Mulherin refers to next. While the scanning routines would be moderately complex, the statistical calculations are straight foward.