If you are selling printing as a service, the cost of ink is an important variable. If you are selling prints it isn't - or at least shouldn't be.
In either case, given the difficulties that exist in micro-measuring consumption, the total inventory approach is really the most practical way of understanding ink costs. We do have total control over knowing the number of square feet of ink coverage we have commissioned from our printers. Keep a record of that in real time. We do have control over knowing how much ink we have bought and consumed in total for the concerned printer - keep track of that, using the information from the printer for an estimate of ink remaining in the tanks. Subtract the remaining ink in the printer and in inventory from the total starting and purchased volumes, and we know reasonably well how much ink we have consumed. At some point after quite a few square feet have been printed (I recommend several hundred), divide the square footage printed into total net ink consumption and you have a decent measure of ink usage per square foot, combined for printing and maintenance. Since both are essential for producing a print, they don't need to be separated to understand total ink cost per print.What we then do with that information I don't know, unless one is printing as a service - in this case it gives decent insight into that element of the cost structure. For people selling prints, other factors will be far more important than the cost of ink.