Hi Eddyfin and welcome. You've come to the right place. How old is your cat and when was he/she diagnosed? Which insulin and dose are you giving? What food are you feeding? It will help to put that information in your signature on your profile page.
Home testing is a must while giving insulin. There are lots of great tips, videos and suggestions on the home page and from members here.
You'll find testing at home will likely result in lower BG (blood glucose) #'s than at the vet because cats tend to be stressed at the vet.
Home testing is very important to determine the correct dose of insulin to give and to help monitor BG #'s from going too low or too high. We have many expert members who will be able to assist you.
You'll also find by home testing and doing curves, you'll be saving money and time by not having it done at the vet and you'll be getting more accurate BG numbers.
Another important must is diet. It is crucial your cat is on a low carb wet (canned) diet under 10%. Here is a link to a cat food nutritional chart:
http://www.catinfo.org/docs/FoodChartPublic9-22-12.pdf
If your cat is eating dry food, now is a good time to wean him/her off the kibble. Click on this link to find out about transitioning a kibble addict off dry food (look in the right-hand column):
catinfo.org
Ask any questions; we're here to help.