Read formula again. ideal pixels of two lenses x (vr.PixelDensity)² = pixels for drawing.
or for the VIVE PRO vr.PixelDensity = sqrt(4608000 / 2x1440x1600) = sqrt(1) = 1
4608000 - all pixels which vive pro has.
“best” value for graphics depends of you hardware specs and scale of oversampling