Optical Axis?

I just ordered an KRISS and am torn between an EOTech XPS and an Aimpoint T1 Micro.

I'm leaning towards the Aimpoint T1 with a 4MOA dot but I don't know which mount to get. Aimpoint has 2 spacers available. The Micro Low Spacer raises the optical axis of the T1 to 30mm (1.181"). The Micro High Spacer raises the optical axis to 39mm (1.535").

What is the optical axis that I am looking for? What is going to allow me to use the BUIS?

On the KRISS USA homepage they show an Aimpoint T1 on what appears to be the Aimpoint Micro Low Spacer. http://www.kriss-usa.com/

If that is the case, then it looks like the American Defense CAS-V T1 mount might work: http://www.americandefensemanufacturing.com/view/product/46/

According to ADM the CAS-V puts the optical axis at 1.155" or 29.3mm. Does that sound about right? How will that line up with the BUIS?

Sorry for an answer that is not a direct one, but the closest I can tell you is that the KRISS BUIS are designed with a co-witness with an EOTech 512 in mind. Maybe ADM is familiar with the optical axis height of the 512 to give you better guidance. Or the L3/EOTech folks directly?

Here is the insane URL for the latest BUIS: http://store.kriss-usa.com/vector-flip-up-sight-system-kit-custom-flip-up-lock-down-operation-fully-adjustable-windage-and-elevation-co-witnesses-only-with-eotech-512-sight-recommended-by-kriss.html

I used a ADM PICATINNY QD MOUNT along with an aimpoint micro. Works perfectly for a co-witness mount.