Morning.
The most important thing is to be consistent, especially in a cat who is sensitive to the insulin. One drop can make a difference.
So what you call 0.75u today, you need to do your best to give the same amount at the next shot too.
Some times the amount we are writing down is what we estimate is a .40.
Many of us have strong reading glasses or magnifyers ( or in my case both) and we really try to know that mark of where we load the syringe.
There are many of us who even use calipers because the markings on the syringes can be inaccurate , even in the same box.
Humans don't need such accuracy as one drop doesn't make such a difference but cats are small.