KF wingfoil (
Kline–Fogleman airfoil ) The purpose of the step, it is claimed, is to allow some of the displaced air to fall into a pocket behind the step and become part of the airfoil shape as a trapped
vortex or vortex attachment. This purportedly prevents separation and maintains airflow over the surface of the airfoil.
- and seen here in this file, they say % of the cord on many of versions...
In FT article this is written : "Note the percentage thickness refers to each sections total thickness vs the total chord of the wing, and not the height of the step. The height of the step is only determined by the thickness of your foam sheet. Steps can be raised to give more height and the step effect increases proportionaly with both height and air speed."
https://www.flitetest.com/articles/kfm-wings-a-basic-explanation