Finishing
The grade is a count of test liquids above the surface tension. The rungs get closer as the ladder rises.
— AATCC
Highest test liquid the surface repels
AATCC 118 is a pass or fail against each liquid in turn and the grade is the highest one that does not wet within thirty seconds; it is not a measurement of critical surface tension, and deriving one from the other in either direction is an inference. The inference is sound in principle and rough in practice, because the standard also depends on how the drop is placed, on surface roughness and on the presence of any hydrophilic contaminant, all of which move the observed grade without moving the chemistry. The Zisman relation is a linear fit of the cosine of contact angle against liquid surface tension, and its whole construction puts the contact angle at zero when the two tensions are equal - which is why a liquid only just above the critical value gives a small angle and a marginal pass. Read a small contact angle at the target grade as a warning that the grade will not survive a soiled or abraded surface. Textile surfaces are also rough and porous, so the wetting they show is Cassie-Baxter rather than the ideal Young behaviour Zisman assumes; roughness helps a repellent surface and hurts a wettable one, which is why a napped fabric can grade above a smooth one carrying the identical finish.
Oil Repellency Grade from Critical Surface Tension — free, with the formula and a worked example, at Textile School.