Bob...
Points of clarification. The location where the buddy surfaced (away from the line) was not very far from the boat. Easy swimming distance. The buddy was however, in a panic due to OOA and the Zodiac picked him up. There was NO current and the viz was 50-100' I have dove Farnsworth 5 times now and each time it has been different. This particular day though, was the best conditions I have personally seen.
As to your air management formula. Great in theory, but, each diver is different in physiology and consumption rate. You need to be aware of your consumption rate and plan/recalculate based on changes in that. My computer tells me time remaining based upon the changes throughout the dive. For example. I did a very similar profile on that same dive...down to 130 Chased my zero remaining bottom time up to 60 feet, did a 2 minute deep stop which ended with 850-900 PSI, slow ascent - not on the line (I could see the boat from 60 Ft) (30 FPM) did a full 5 minutes at 15 feet and still landed on the back deck with just a bit over 500 PSI.
I have to agree with several who have said that Blue water, free accents can be VERY stressful. (Been there done that, have the shirt) But, if properly managed/practiced, they are relatively easy. I have done many in Murky Green Northwest waters with less than a foot or so of viz. I use my SMB, and make a slow casual ascent up my line.. But, if you haven't practiced it and have to do it on the fly, well, big time stressor. Additionally, the boat had and deployed their zodiac. And regularly briefed that "If you are having issues, signal and we will come get you" Which is exactly how it went down.
Not sure the buddy team was an issue either. The split was VERY short duration...Pop up take a bearing and descend.
I am still pondering weight ditching. What is it that causes a diver at 15 feet to keep descending and NOT ditch all weights in an Emergency. This is the second recent case of this that I am aware of. PADI has changed their Curriculum and Standards to focus and emphasize the ditching of both belt and integrated systems. When I do refreshers or help with classes I always ask "If you were OOA at the surface or near the surface, how could you become more buoyant. I am surprised by the answers I get at times. #1 answer I hear is "Orally Inflate BC"
If nothing else, I hope this accident reinforces instructors and AI's, DM's to teach and reteach weight ditch and other skills (SMB deployment, air management, Nav) And also take the time to really look at a divers ability and feel compelled/obligated to point out flaws or errors in a divers planning, practices and logic. Also for the Non Pro divers; Take time to PRACTICE!!! SMB's Nav, Buoyancy, OOA EP's etc etc.
~R~