Potatoes in bread? How to estimate potatoes needed?
You'll want to be careful when trying to modify an existing non-potato recipe. 

Potatoes, like any other agricultural product, have a variable amount of starch and water. Some are big, some are small, etc, so it's hard to tell just how much flour and/or water you'd want to replace in an existing recipe without doing some serious lab-testing or trying to remove all the water somehow.

The Non-risky method:
Look for published recipes that already have potato added:

http://www.thefreshloaf.com/search/node/Potato

The Experimental Method
Take a recipe you like that could use a bit more moistness. Throw in a cup of baked potato. Remove a half-cup of flour and 1/4 cup of whatever liquid you're using.  You can also try using Potato flour: 

http://www.bobsredmill.com/potato-flour.html

Bake that loaf and see what happens. Adjust the variables as necessary, and try it again, carefully documenting what worked and what didn't. If you develop something that you like, publish it so the rest of us can enjoy!