Your post indicates "a portion of the home" is being rented out.

This can make the allocation much more complicated.

Assuming that portion is rented out (or available to rent with no personal use), then the allocation is NOT neccessarily based on days but another reasonable method.

Part of the issue is allocating the expenses between personal and rental use.  If a portion of home rented is 2,000sqft and has 4 people, while the owner occupies 500sqft and is 1 person, you certainly do not want to use days.

That could include SqFt, # of People, or other reasonable method.