Thank you all. Yes, I do have busy brain. Clearly, I need to work on my breathing topside. Also, lose some lead and swim more efficiently. Not all that easy to work on this an accrue more dives as we live in Chicago. So, there's the quarry....
I would not worry about it. I have noticed that, even in my very limited experience, that your air consumption will change dramatically as you get more comfortable underwater.
Example (all similar profile dives to about 10-12m with no currents to speak off and same gear ie wetsuit, tank, fins mask etc):
OW check out dives - 12l tank @230(ish) bar =approx 30-35 mins.
Few weekends later on a trip with exactly the same gear as above and same tanks
Dive 5 - 12l tank@ 230 bar =35 mins (was actually a deeper profile to about 18-19m)
Dive 6 - 12l tank @230 bar =45 mins
Dive 7 - 12l tank @240 bar =25 mins (no end of issues before the dive so was stressed getting in the water and took about 5-10 mins to completely relax)
Dive 8 - 12l tank @230 bar = 40 mins
So from just getting over 30 mins from a tank I went up to 40-45 mins (bearing in mind the deeper depth of the fifth dive would up my gas usage.
Only differences between dives - how relaxed I was. Dives 1-4 (OW) were stressful due to exam conditions and doing skills, dive 7 was stressful due to equipment issues whereas dives 5,6 &8 were easy going dives with no stress.
Sounds daft but concentrate of being relaxed and smooth instead of worrying.