igoRluse:
By what I have understood, correct me if I am wrong, when using a SCC with eanx you would have the same nitrogen absorption as using the eanx in open circuit. From there I imagine, you would simply use a ntirox capable computer...
Am I correct?
Not quite. When you dive OC you breath the fixed percentage of O2 in you mix. So you set your nitrox computer to that mix, and it will calculate accordingly.
When diving a rebreather the mix within the loop changes. Even with a SCR that uses pre-mixed gas. You metabolize gas that is in the loop, and that amount will vary depending on a variety of factors like you physical condition, your workload etc.. In a cmf-SCR like the Ray gas is added continiously through the metered orifice, and the bypass as needed. They can be 'overbreathed', meaning you metabolize more O2 than is added. Which is why I second landfish's statement that you should should dive with pO2 monitor.
Now in regards to the deco computer, if you use a computer (or OC tables) you have to calculate the worst case scenario. To figure out the O2 toxieties you use the highest O2 content (mix in tank), for deco the highest N2 content (when you 'breath down' the O2 level considerably and thus end up with a higher N2). This will give you safety, but obviously will be very conservative as you can't really take advantage of the actual mix in the loop.
If you add a pO2 monitor such as Dräger's Oxygauge, you will be able to watch your loop content, take notes and program your fO2 computer accordingly (though you still need to be conservative). You will also have the added benefit of high (depth) and low (overbreathing) alarms.
I think the better alternative is to use an O2 cell integrated computer that measures the O2 contend, displays it, and uses it to calculate your deco on the fly. The Ray's P-connector that allows you to install the Oxygauge will also accept the cell holder for the DeltaP VR2, a nitrox computer with 4 mixes OC, semi-closed or closed circuit. (As well as the VR3 and HS Explorer, two cell integrated 10 gas trimix computers, though they are overkill for a Ray).
Another possibility is the use of Uwatec's ZO2 computer with its Oxy2 SCR interface. The Oxy2 has two redundant O2 cells and screws between the rebreather and the inhalation hose. For the Ray you need an adapter as it was designed for use with the Dolphin. The pO2 is transmitted wireless to the ZO2 wrist computer. The ZO2 displays both your pO2 and the derived fO2, and uses the actual mix in the loop for its calculations and alarms. The ZO2 can also be used as a single mix OC nitrox computer, with or without wireless contents display.