For those interested I spent several hours tweaking the grayscale on four picture modes (Orcus Natural, Orcus Digital Cinema, Neilvm Custom Gamma & Neilvm -2 Gamma) to see how accurate I could get the grayscale with only adjusting the two point controls.
I also found that adjusting the two point controls also effects the brightness & contrast controls so there was lots of checking the black & white clipping patterns, measuring & adjusting the two point controls & then rechecking the black & white clipping patterns to see if they had been adversely effected & if so adjusting them & then rechecking the grayscale.
The errors show in the images below are going to originate from differences in projectors from unit to unit, screen materials & ambient light conditions, but as you can see great improvements can be made in the greyscale with a little tweaking.
Neilvm Custom Gamma: Default Settings
Neilvm Custom Gamma: Two Point Adjusted
Neilvm -2 Gamma: Default Settings
Nielvm -2 Gamma: Two Point Adjusted
Orcus Natural: Default Settings
Orcus Natural: Two Point Adjusted
Orcus Digital Cinema: Default Settings
Orcus Digital Cinema: Two Point Adjusted
After viewing content over all four modes I prefer Neilvm's custom gamma settings & Orcus's Natural settings.
Even after tweaking the two point I still feel that Orcus's digital cinema looks very green but I did not have time to have another look at them again but I will be honest I probably wont use it due to the drop in light output from the filter being placed in the light path away as his natural mode is much brighter for me.
The results probably are not much use to anyone except me really as they are only for my room, projector & screen but its interesting to see how much error can occur with these factors at play.