Part of me says “I’m not here to do the math for you dude” and the other half says “well someone might learn something in reading this train wreck.”
The thread has flipped from metric to imperial and back enough times, here’s an example:
Sheer **** hits the fan math at 300’ let’s say 2cf/min equivalent for 5min before you begin your ascent. 10ATM * 2cf/min = 20cf/min * 5 = 100cf used before ascent. Pair of American LP50 are closer to 8L but we’ll stick w them bc they’re the GUE normal. 3600psi/2640psi*50cf/1.15 compressibility factor*2bottles = ~118cf.
Toss this 300’ 30min in the ocean plan in (which is a shitload of dive in the open ocean) and a quick ascent to your 21/35 or whatever intermediate deco and you get the attached profile of 50/70.
Let’s bring this back to your 200’ profile and get something like the second attached image (and then add another 70cf of bottom gas used for the 5min CO2 brain dead idk what to do, can’t ascend, blagh blagh panic).
You do you, but I’d guess most of the people in this forum w extensive experience w profiles yours and longer or yours and deeper would add a scooter, backmount a pair of LP50s of 18/45 (or even maybe add a few more bips of helium), sling an 80 of 50% and (for the v conservative) a lightly filled 40 or 80 of oxygen and call it a day.
Sources and assumptions: math is above, have had a CO2 hit deeper than 60m, my sac rate in this dive planner is accurate and low, and really don’t want people pretending bottles on their back that they can’t see are remotely safe. Other than my Neanderthal 5min panic assumption, your limiting gasses in open water deep dive will be deco gas volumes, not backmount gas volumes, esp in this range.
