How to divide common area expenses

[Lucas Hammer]

I just closed on a 2-flat that I'll be living in and renting out the second unit. I'm tracking all of my receipts currently and trying to categorize them as well as I can. I've read IRS pub 527 a few times and I'm wondering if I can just divide all of the expenses in half. 

My unit is slightly bigger, but has the same number of bedrooms and things like foundation and yard work affect both units equally. Is that considered a reasonable way to divide up the units? It seems reasonable to me, but the last thing I want is to end up on the wrong side of an audit my first year in REI.