My process is similar to Terry’s. But I have 1.8 reservoir which lasts me about 1 week. So I do insulin about once a week and new infusion set about 2 times a week.
I never put insulin back into bottle from a used reservoir or add insulin to reuse a reservoir
However, when time to fill a new reservoir but not enough insulin in the current bottle. I will do the following.
Estimate how much more I need in old bottle. For me, with bottle inverted, it means up to where the label is.
Then with new reservoir, take out some insulin from New bottle, and then add that to old bottle.
Then do complete fill from Old bottle, which should use all that is there.
This prevents any contamination of new insulin bottle, and minimal amount left in old bottle to throw out. And always starting with a full reservoir each time.