my understanding is double would be 10mg at next scheduled dose  and 15mg at next dose for triple.  my sons endo has only told me to triple he gets 1.5 ml at 6am 2 pm and 10 pm so if he is running a fever of 101 or higher i give him 4.5ml until he has no fever w/o med taking the fever down. and he gets the stress dose for 24 hrs after the fever is gone.  we were given a book about cortisol therapy from pur endo it is very good for defining when and how to stress dose.  it is called CORTISOL REPLACEMENT THERAPY  an education booklet for parents and children. it was developed by Judith J. Henry,R.N.  in conjuction with the pediatric endocrinology nursing society.  it also has instructions in there on how to give the injection.  i found this book to be helpful.  there is even spaces for you to fill your childs doses so you could have the doc fill those in for you and then you wouldnt have to second guess.  hope this helps