15 or 20 is definately too high. I would advise you to start researching how to do carb counting and adjust insulin by yourself. Its not difficult, just a bit time consuming at the beggining. Your nurse probably doesnt know much about carb counting. Have you talked to your diabetic consultant in your clinic/hospital?
If you know how to use insulin your levels will get much better. Im most of the time 5-8. over 10 only if i eat out and not sure how much insulin to take