I use a digital kitchen scale to weigh my cartridges.
I weighed a completely empty one (169g)
A completely full one weights 365g... Pretty close to 1g per ml. I use 220's mostly
I put a label on each new cartridge and document the purchase date, the install date, the starting weight, and then I weigh them whenever I am curious where I stand.
Here is the thing...
The printer does not give a crap how much ink is actually in your cartridges.
The printer talks to the smart card on the cartridge. It then deducts ink from the total based on how many pump cycles and print parameters (like dot size, wave type, etc..)
When it calculates you should be empty the printer pronounces the cartride dead. No matter how much ink is actually still in it.
If you have a bad capping station then the pump thinks it is pulling ink to do cleanings, but it is really sucking air. It deducts the ink from your smartcard, but the ink never left the bag.
You will need to call your vendor and get some replacement smart cards when this happens so you can reclaim the still-full cartridge.
I do not replace the cartridges until the printer stops printing. Then I pull the dead cartridge and the printer starts going again using the ink from the sub tank (I have a 1604, with 150ml subtanks) Then I replace the cartridge when that job is done in case I get an "not original ink" error when inserting the new cartridge.
Tip:
Keep at least one full set of ink cartridges AND a full set of replacement smart cards.... Cartridges always seem to die on Friday night at 6pm for a job due Saturday.