The ideal answer is all of the meal should be eaten before the shot is given. When you give an insulin shot before the pet has eaten, you are "betting" that he/she will eat all of it. Since the number of units of insulin is dependent on both body weight and the amount of food eaten, you could be in serious trouble with hypoglycemia, or low blood sugar, because there's a bad balance between food eaten and the amount of insulin.
Intervet Page 15 advises a basal (body's needs without considering food eaten) dose of approximately 30% of normal insulin dose when a diabetic pet isn't eating. This should be discussed with your vet even if you do not need the information at present. Knowing what he or she considers to be your dog's basal insulin dose should be noted in case of need.