Usually, when your insulin has gone, you will see it in your numbers.
Since you have only recently started your spreadsheet, you can't really look at how the numbers were before but from the looks of it, you are still getting a good reaction to the insulin.
I call it 'THE JOJO SQUINT' ... if you look at your spreadsheet and start to see the colors slowly going higher with each dose.... less blues and more yellows, then the odd pink number shows up, and then you have no more blues, well, if you see that your Lantus bottle has been in use for over a month, it's likely time to crack open a fresh container of insulin.
The 28days is like a guarantee; that all testing has shown the lantus to be good till that date. Sure, it could last longer, but the testing, the guarantee given by the mfgr goes to only 28 days.
Levemir is said to be good for 42days, but it's lasted longer for many people.
You are being told that your insulin should definitely be good till 28days or so, but nothing is to stop a vial from being bad in a week! For sure you would return it, but a vial that seems to have fizzled after 34 days, well, it's past the 28 so time for a new container.
The vials are 10ml and if you are on a small dose like you are, you will end up wasting more than you use, so tell your vet you want the next rx for pens of cartridges as they are only 3ml each.