Hi and welcome. Already you are on the right track by discontinuing the dry food and limiting when you feed. And you on the right track for promptly noticing the large volume of urine.
You'll want to aim for low carb wet/canned food under 10%. You are already aware that you don't need an expensive prescription diet to manage your cat's diabetes.
Here's a handy cat food nutritional chart:
http://www.catinfo.org/docs/FoodChartPublic9-22-12.pdf
You'll notice as the diabetes is better regulated, the amount of urine in the litter will be less. CJ used to gulp down water like crazy and leave boulders in the litter box.
When I was injecting CJ with insulin, I injected in her neck scruff daily without a problem. I often would just do one side in the am and the other side in the pm. The needle is so thin she never felt anything. Sometimes I'd worry I didn't get any insulin in because that's how thin the needle is. There were times when I'd inject in CJ's side if there was enough skin and "flab" to pull up but I personally preferred aiming around the neck area.
Home testing is a must to ensure correct insulin dosing and prevent hypo episodes (glucose levels going too low). The cat's ears have few nerve receptors so therefore poking isn't painful for a cat. The more you test, the more capillaries that grow in the ears, making drawing blood much easier. Start out by massaging/touching your cat's ears a lot and rewarding with treats and praise so Tryone gets used to it. Then graduate to the next steps with the tips and videos in the link below:
http://www.felinediabetes.com/FDMB/threads/hometesting-links-and-tips.287/
You'll also get great tips from members here in the forum.
We suggest starting out with a lower # lancet/gauge such as 26 or 28 rather than the higher, thinner 30 or 31 gauges that come with the meters. To warm up your cat's ear to get blood flowing faster, a lot of us have several tricks; you'll find one that works for you. I use a terrycloth headband or small washrag, dampen it with water, microwave it very briefly and then place it in a ziplock bag. I test it on my arm first to make sure it is warm, not hot. Then I place it gently and briefly on CJ's ear to warm it up. To bead up blood, a trick is to apply a very tiny dab of ointment with a q-tip, such as vaseline, neosporin or polysporin. Be sure to use a very tiny amount; otherwise it will create an error message on the glucometer.
Others find warming up rice in a sock helps. You'll find a technique that works best for you.
A key is to aim for the "sweet spot", the area between the vein and the outer ear. A handy picture of the sweet spot is in the first post in the link here:
http://www.felinediabetes.com/FDMB/threads/testing-and-shooting-tips.85113/
If we all knew beforehand what to do to prevent feline diabetes, we wouldn't be here! No use feeling guilty. You're doing all the right things and are in in good hands at FDMB.