Hi
I think youve got yourself confused with infusion set (cannula) and the cartridge which holds the insulin.
What youve got to try and do is fill the cartridge up with enough insulin to last James the 6 days and add on about 20u to deal with priming the tube and add on another 5u to deal with priming the infusion sets. That way there will be a few units spare to be enough incase James decides to eat more carb for some of his meals or decides to use a TBR. At the moment I think the dsn just wants you and James to get used to changing cartridges and inserting infusion sets after 3 days so with that in mind, you will only need to draw up 3 days worth of insulin plus the extra for the tube etc. Once you get used to filling cartridges and inserting infusion sets you can do them seperately or do them together. Its up to you to do what's convenient to you and James...
With regards to when you fill the cartridge etc..... its entirely up to you really. I sometimes do cartridge changes at night when there's no more insulin left or if there is enough to get me through me the night, I do it in the morning. If my infusion set has only been in 1 or 2 days I will leave it in until the 3rd day even though Ive replaced the cartridge the day before.... it doesn't matter..