I'm renting out part (a studio room) of my house. Do I use the full house value to calculate original purchase price, fair market value, land value, etc.?

Yes, you use the full value and the proportion of the value based on the space that is rented.  

You can use a method based on the number of rooms like you described in your example.  Typically a more accurate calculation would be based on the square feet of the space being rented versus the square feet of the whole house.