Hello Darci and Ringo! Your vet sounds on top of it. Ringo's BGL is high, but not super high. So, your vet has started you on a low dose of a good insulin for cats.
What has Ringo been eating? The best diet for a diabetic cat is low carb, high protein canned food.
There's a great chart here with choices. The thing is, it might be very worth it to try to change Ringo's diet first, before beginning the insulin, if you feel comfortable delaying the insulin for a few days. It's possible for the diet switch alone to drop his BGL over 100 points.
Be aware that if you do start the insulin, and you change his food at the same time, you will need to monitor his blood glucose level carefully, to avoid a dangerous hypoglycemic event. In either case, home testing Ringo's BGL is the safest way to monitor him and keep him safe. There's no need to buy an expensive pet meter (the meter is cheap enough but the test strips are more pricey.) Many members here use an inexpensive human meter, such as WalMart's ReliOn. Very cheap test strips.
You are on a great site to find out more about feline diabetes, with some very experienced members ready to help. A great place to start is this thread:
New? How You Can Help Us Help You!
Edited to add: Diana is right about starting the insulin. I was going by the number in your post, and had not seen the numbers in the attachment. Some people do decide to switch the diet completely before starting insulin, to protect against hypo events, but starting immediately on insulin is also perfectly advisable, as long as you home test the cat's BGL to be safe.
