You have several choices.
1. Skip the shot. This is the safest. Your cat will not have their BG level drop too low with possibly too much insulin.
2. Give a reduced dose. Perhaps half of your cats normal dose. You should still monitor closely with home testing to know how your cat is doing.
4. Stall the shot. By this we mean, test, do not feed, do not give the insulin yet. Test again in 30 minutes. If the BG number is now over 200, it will be safe to give the insulin and feed.
If the BG is still below 200, wait another 30 minutes without feeding and retest. Over 200? then feed and shoot.
Under 200 delay food and insulin again.
You will reach a point in this stalling method where the BG number is either high enough to shoot or too low to shoot and you will need to skip the shot for safety.
It is easier to know what you should do if your cat is low once you have done some testing and gathered more data on how YOUR cat reacts. Every cat is different.
You can always post on here and ask for assistance. We will give you recommendations based on your spreadsheet and the numbers that you have gathered in testing. If you do not have enough tests yet, we will tell you to skip the shot.