If you're strictly cal counting then weigh your potatoes and check an online cal counter for the nutritional breakdown. Add anything you add to your potatoes too (e.g. butter, milk etc.)- You can get cheap scales in £-stretcher or Argos for around £5?
I check the cal content of each ingredient in a meal and add it all up. Most 'packets' of food these days give the cal content of the packet which is handy. HTH?