The Fancy Feast canned food is good if it's pate, no sauces or gravy. Dry food is too high in carbohydrates for a diabetic cat, and it's best to slowly transition him off to an all canned food diet.
Here's the basic testing routine we recommend: (do what you can on work days - before bed is useful along with the essential pre shots)
test every day AM and PM before feeding and injecting (no food at least 2 hours before) to see if the planned dose is safe
test at least once near mid cycle or at bedtime daily to see how low the BG goes
do extra tests on days off to fill in the response picture
if indicated by consistently high numbers on your spreadsheet, increase the dose by no more than 0.25 u at a time so you don't accidentally go right past a good dose
post here for advice whenever you're confused or unsure of what to do.
It's time for you to visit the forum that has information about the insulin you have been using:
http://www.felinediabetes.com/FDMB/forums/caninsulin-vetsulin-and-n-nph.19/ and read the information here:
http://www.felinediabetes.com/FDMB/threads/beginners-guide-to-novolin-humulin-nph.186097/
I have not used this insulin and cannot advise you on how to use it, or dosing.
I think Sneaky's blood sugar went way too low and has been going too low since his dental extractions. It may be that he needs much less insulin now, and the only way you are going to know that is by testing him, often and regularly. Shooting blind (not testing) is very dangerous. You thought he was dying yesterday and possibly this morning too. I don't think you want to repeat that, and to be honest I don't know how many times Sneaky can come back from that.
You will need to make a spreadsheet and record your testing data so you can see the big picture. If you aren't able to do that on your device (what are you using, a laptop, tablet or phone?) someone can set that up for you.
http://www.felinediabetes.com/FDMB/threads/fdmb-spreadsheet-instructions.130337/
http://www.felinediabetes.com/FDMB/threads/understanding-the-spreadsheet-grid.156606/