Leather drop-in lining

I would like to know if there is a rule out there for sizing a leather drop-in lining. Do I use the same measurements as the outside leather (I think this might make it too big) or do I remove a number of millimeters all around? Measuring the bag when it is stitched up isn't really going to be easy as there isn't a single straight line in it and it would leave too much uncertainty. I haven't been able to find a definitive answer to my dilemma anywhere!!