Doing a test before EVERY shot is the only safe way to know if the numbers are safe to shoot. Glucose numbers can change as the body changes...you may need to use more or you may need to use less...there is no way to tell unless you are testing. A good comparison is when a person is diagnosed as diabetic and started on insulin, they are TOLD to test before each shot and test in between. If a person is dropping into too low of numbers they can tell, since they feel "weird". There is no way for our kitties to tell us so we need to test to keep them safe. Unfortunately the old school way with vets was periodic tests and curves at the vets. The new standards are to encourage regular home testing and have the curves done at home.
As far as the dose...I really can't say if you should give the same dose without knowing what the preshot test is.