Start home testing before you make any diet change. The change to low carb wet food can have sometimes dramatic effects in lowering their insulin needs and BG values. And you need to stay no your toes when you do that. Frequently [because of this potential for change], it is recommended to drop back to 1u twice per day and re-regulate the cat once the food change is made. My cat's BGs went down at
least 200 points from the food change. The food change will allow the diabetes to get under control faster - some cats go into remission [needing no insulin] just from the food change.
IMHO, the diet change to low carb wet only needs to happen first, then you can figure out how much your cat should eat to lose weight [safely] and only give that amount - calorie restriction. Personally, I'd just change the food and wait to see the effect of that on the curve - it usually starts to happen within seven days. Then you can reduce the amount needed for the kitty weight loss diet [I would recommended doing this slowly ever days]. And you will also likely see the effects of that on the BGs too because food in = BGs up. Or in this case less food in will likely mean slightly lower insulin need. You might want to wait too for the weight loss [calorie restriction] diet until you have your kitty regulated because not having things regulated plays tricks with the metabolism. As hinted at here they can eat like crazy and still lose weight if unregulated. And while the are unregulated it is often recommended to give them as much food as they need. Ask you vet and ask around here too about how they would time the calorie restriction with the diet change.
Dr. Pierson as a bit on feline obesity on her web page here:
http://www.catinfo.org/?link=felineobesity
The main thing to a kitty 'diet' is that they are not supposed to lose more than a certain % of their body weight over a given period of time. If this is not on Dr. Peirson's page or you cannot find it elsewhere, please let me know and I'll try to dig that up for you.
One can use something like the SS I made here to come at some rough claculations as to how much your cat should eat:
https://spreadsheets.google.com/ccc?key ... nhPTXBEUUE
You have do download as (excel) then upload or import back into google if you wish to use it in google.
Likely you will need a scale. I'm not sure how much your cat weighs but if under 16.5 or 17 lbs then this is a great scale:
http://www.google.com/search?q=my+weigh+kd-8000
If over than than the My Weigh also makes the
http://www.google.com/search?q=my+weigh+wr-12
The scales also help you weigh the food to make sure they get the right amount.
Folks here that have "hoover" kitties frequently find the timed feeders help spread out the meals during the day to help reduce the effect of food on the BG curve. The 5 slot ones I think are the best. Nancy of Nancy & Cody has tip on that and you can PM her if you wish. Many others here have ideas about timed feeders too - no shortage of advice on that topic around here.