i think we can conclude this topic as i found a reference from IRS website, and here is my takeaway:
1, for some utilities it maybe able to argue divide by number of people in use, for instance 2 people in my case for water bill, electricity/gas bill (w/ assumption tenant had full run in common room), internet bill (2 people share).
2, while for other expense it would be best to only use square foot method, like insurance, property tax, mortgage interest, because they are related to rental area.
3, for depreciate, no question to use square foot method.
so i hope turbotax can make it more flexible for utility bills here in next version, otherwise modify manually later is not a big deal as well.
How to divide expenses.
If an expense is for both rental use and personal use, such as mortgage interest or heat for the entire house, you must divide the expense between rental use and personal use. You can use any reasonable method for dividing the expense. It may be reasonable to divide the cost of some items (for example, water) based on the number of people using them. The two most common methods for dividing an expense are (1) the number of rooms in your home, and (2) the square footage of your home.
