Grab Pass - VR Single Pass Coordinate calculation bug

https://github.com/kalineh/unity-grab-pass-single-pass-bug

float4 scaleOffset = unity_StereoScaleOffset[unity_StereoEyeIndex];
grabUV = (grabUV - scaleOffset.zw) / scaleOffset.xy;
grabUV.x += 0.06 * unity_StereoEyeIndex;
grabUV += _BackgroundTexture_TexelSize.xy * 0.5;​

See sample scenes. Each material attempts to find correct grab pass coordinates according to documentation / internet searching. Final material SP4 seems to work but there is a hardcoded x += 0.6f which is reported to be different per headset. Cannot find how to correctly calculate UV for grab pass screen coordinates in VR Single Pass!

The documentation on VR single pass coordinates and the macros/helper functions is really hard to navigate, I can't seem to find clear examples on correct intended practice for these cases. As I sort of understand it's roughly these:

a) regular VR render (nothing special needed)
b) stereo render (vert/frag UNITY_STEREO macros for worldspace rendering)
c) screenspace effects (UnityScreenSpaceAdjustUV, etc)
d) grabpass (???)

In any case, this hardcoded offset doesn't seem to fit and I have no idea what a correct shader for VR Single Pass Render + Grab Pass is supposed to look like.

bug reference: https://fogbugz.unity3d.com/default.asp?975537_3b6e3drr1qvcobl1

 

I reported the same issue, bug case: 968570