So you blow thirds if you have a spare scooter but follow thirds if you don't?
You start with 3000 psi, you turn at 2000 psi. How much gas do you have when you turn (2x what you used getting in is 2x to get out).
All this obviously assumes no siphons, and are in caves that aren't like Cathedral where gas planning gets adjusted based on conditions, no different than the rule of thirds not being applicable in those kinds of caves.
1.5x the gas required to kick out when you are going in on a scooter ends up somewhere around sixths for the most part with my scooter because it is a UV26 and is slow.
Assume a 150fpm entry, starting at 3000psi, and a 20 minute penetration for shits and giggles, and you turn at 2500psi. 25psi/min entry. 3x the required exit time puts you at a 60min exit, so 25*60=1500psi. 2500/1500=1.6x the gas to get out kicking.
If you are going in on a faster scooter, you have to make that more conservative on your entry, which you're well aware since you have a go-fast DPV.
I don't know anyone who turns at thirds when on a DPV with a planned kick exit. Practically speaking, if you have a spare DPV, you can cut your exit gas requirements since a silt out is about the only thing that will cause you to not be able to go out at some sort of speed. For this, I plan 100fpm dpv exit speed at a 2x gas safety factor, which happens to equate to 1x the kick out gas requirement. There is a minimum of 300psi buffer in there because I deduct that from all of my tanks before running calculations. I'm OK with this because the odds of you not being able to dpv out at all for the entire length of the cave so long as we are talking about the caves above and not special circumstance caves.
I wrote a spreadsheet that has this math calculated out for me so I can enter some variables that are applicable to the cave. Planned depth, how long I plan on being on the DPV, how much time I plan to kick around at the end, my planned speeds going in and out, SAC rates going in and out since I acknowledge they are usually different, and then whatever safety factor I want to apply. Caves like JB, Ginnie, etc. I'm OK going at 1.5. Caves like HITW will get bumped up to 1.7-2.0 depending on where we are going because it is no flow, with high risk of getting silted out if another team comes in behind us or we mess up on the way in.
You then input what backgas tanks you're carrying and it will also spit back out the minimum number of stage bottles you have to bring with you which is quite convenient.
This tool is great when planning dives with decent maps when you haven't been able to dive them before and while it has to make quite a few assumptions, it has yet to play out less conservative than real world validation because the numbers are all padded pretty heavily.
I personally find diving stages to 1/2+2 and reserving backgas to be significantly easier than diving stages to thirds, so the spreadsheet is set up to factor that in. Obviously adjustable to whatever you want based on how the formulas are set up.
The next tab over is for CCR bailout planning which is identical other than removing the entry gas requirements. The top left is DPV penetration, and then kick penetration since the b/o requirements for those two legs will be different.
I don't use straight cut rules because it will get you offed, but after doing the math, some generalizations do start to become apparent, one of which is sixths being OK to dive to when diving with slow scooters.
