I use rooms used for daycare Vs total amount of rooms in the house since I think this is a less complex approach. All daycare spaces are dedicated, so I do not have to calculate hours used during the day. I do not consider halls, stairs, etc.

You can also calculate % of square footage. If you want to be very accurate you could do what your are thinking on doing and measure all rooms, halls, etc. and add approx. hours of transition spaces usage to your formulas