For open-water tech dives a rule of thirds might be overly conservative for the back-gas.
Assuming a diver needs donated gas at depth, she can still switch back to her own stage at the first gas switch.
So for this dive we’d need to calculate MG from double SMRV, with time for problem solving at depth, ascending and switching gases.
If this is complicated to program in, another option is to have this as a scenario/modifier feature. Like the stress SMRV for the whole ascent, except it’s just up until the gas switch
