I don't think that the 12hrs has to be exact. I fasted Shadoe because her tests mentioned lipemia, and she has that value raised in all her bloodwork. I cut her off at midnite then took her in for tests at 8am and she stayed there till the draw and what all was being done.
I always test before we leave for the vet, and also take her meter and strips with us, telling the vet to test her before and after whatever is being done, like dental. For just blood draw, I test before I take her in, wait for her, then test again when we get back home.
For her dental, I left her meter there with her, then when I returned, I had a container of some good foods she would like. She had only a tiny nibble, but I think it is enough to calm her by letting her know I am thinking of her and food is close by.
For her u/s, I think she was too freaked with the shaved belly and all, so she ate nothing.
I took meter and foods for Ollie when he had his Xrays and he GOBBLED the whole can after.
If you want any sorts of numbers, then be sure to leave your monitor and you will have consistency.
What about moving your shot time back by 15min and then you can stick to the 6hrs, but still be able to feed a bit. You still want to stick to no food before the shots, so feeding before will wreck your number.