Hi John,
I agree, it could go too far to establish exactly what it cost and yes, your solution could work. The problem is always that some cartridges will run out before others and the need to change them will displace to calculations.
I suppose I could keep a careful check on how much I actually print over six months and see how much I have spent, bit I have a near-enough guestimate and am not too worried. It was more a question of establishing what it cost me to produce an A4 or A3 print so that I don't give away too much when printing for friends and cover my costs.
I think with the high quality I am able to produce, the cost to cover would be £5 and £10 respectively for friends. If they want cheap, they can go to a Lab.
Have a great Christmas
Malcolm