I would agree with BJM. I started off with transitioning to a lower carb diet first, then started the insulin. I found that there was a drop just from the food change. (Took about a week to 2 weeks to see the difference). Then, when I saw a "levelling off" of her glucose levels from diet alone...the vet titrated what he thought should be a starting dose for her. We went even lower on dosage than his original thoughts as even he re-thought that number.
But, if you're going to be going onto the insulin with a day or two....I'd stick with the regular diet and learn home monitoring first. Remember that your vet is likely dosing, according to the numbers he's seeing right now...based on the food your kitty is eating, right now so, an adjustment in foods would likely bring that number down further and *may* be too much of a reduction/dangerous. Remember that higher numbers are the lesser of the evils at first because too low numbers kill quickly.
So, either start the low carb diet for a week or so first...without insulin and learn home testing methods....or...go with the regular diet and what your vet has prescribed in dosing and see where the numbers go on insulin alone then, revisit with your vet before starting low carb. Remember that doing both at the same time can be dangerous...especially, if you're not doing home testing.
That's just my 2 cent's worth.