any dangerous rise in blood glucose must be corrected. If, 20 minutes after the meal you get a test, if the sugar level is normal, it is dangerous to sting yourself. you risk having a Hypo. When sugar stays too high for an hour or more, there is a risk of damaging your glands. Before a meal, light and with salads and meats, if your blood sugar is already low, there too, it will be dangerous to inject yourself. but in a good habit of care, it's better to have insuline before, the best you can do, is, doing a test before and twenty minutes after you have eating

Check blood sugar before breakfast and inject after breakfast. Try to keep injections at regular times, usualy 12hrs apart.