There's about a 20% measurement error with any test. I wouldn't be terribly concerned about numbers being off by 10 points.
You don't have your signature set up yet. It would be helpful to have that information -- I had to search to see what insulin you're using. This is our
guide to Vetsulin. It would be helpful to know where Felix's blood glucose numbers are at present. In general, you do not want to give insulin if his numbers are below 90. However, at the beginning of this process, you may want to have a higher "no shot" number until you feel more comfortable giving insulin with lower doses. You mentioned a 463 on Felix's initial blood panel. Do you have any additional test data from your vet? If his numbers have been this high, we'll often suggest posting and asking for help if the numbers are under 200.