How do they breathe? The MK19 or MK18 or MK25?
If you would write about 2nds, I would understand.
But 1sts just deliver the more or less (un)balanced and (un)stable IP for your 2nds, from which you breathe.
I think you are mixing something up.
It's not very probable ( but possible in cases of balanced or unbalanced 1sts) that you could practically evaluate any 1st stage by breathing from your 2nd.
You would need at least an IP meter ( or Flow Bench) or extreme dive profiles or situations.
To me the 2nds are basically the key for the evaluation of 'breathing'.
But maybe I miss there something in my English concerning 'breathing'.