I have also never had to give the shot but we are instructed to give 50mg which I believe is worked out on our 2.5 year old daughter's weight - this is in an emergency situation.  With previous episodes of vomitting we have been able to get to the hospital in a relatively short period of time and they have given her the hydrocortisone IV.  I believe in an earlier thread that some parents have been instructed to give the 100mg in an emergency situation.