Personally, I would not give any size dose with levels that low and I would learn to home test immediately. At the beginning, I was following the advice of the vet and shooting my cat with no idea of what his levels were and he eventually had a seizure once the BG levels got low. At the time, he was getting 4 u twice a day. After the seizure, the vet said to give him 4 u once a day. I could not and would not give him the same size dose that caused his seizure. That did not make any sense. Then the vet said that we would have to "expect" another seizure. Whoa!!! Not acceptable at all!! After that, I learned how to test and if I could not get any blood to test, I did not give him a shot. No BG test, no shot became my new motto. He eventually got off the juice after only a few weeks on insulin and a diet change to high protein (which can also lower his need for insulin greatly).
When I had to go to work, I would leave food in a timer feeder so that he had something to eat during the day to help with his blood sugar. It made me feel better that he had something to eat when his BG level may be lower while I was gone. By giving him 4 small meals a day instead of just 2, I think that also helped his pancreas to heal and helped him get off insulin.