If we could do this, we could just allocate sewer costs based on water usage proportion. 

As you scale your business it will become increasingly more difficult to allocate your expenses to the proper rental or business if expenses are running through your personal account.  

If the allocation of income/loss is not based on the capital contribution percentages, it is best to get a professional and draft a  partnership agreement.

Most of the people use the value allocated to the property tax documents. 

You perform a 1031 exchange from the current asset into a bigger asset of the same type within the allocated window allowed by law. (45 days to identify replacement + 180 days to close)C.

Value 1.3 MillionObvious expenses Taxes, Insurance,  allocating 12% to maintenance.

So, my main question is whether or not we are overshooting or undershooting with our 15% budget or should we allocate more since it was build in the 1950s. 

Meaning  If you have a partnership agreement that has special allocation and other terms that more like a business, than this more likely be considered a partnership. 

This will actually give you some room to make a profit.  10% of the ARV is already allocated to selling costs (realtor commissions, closing costs, and property transfer taxes), another 10% will be towards your financing costs and holding costs, and then you'll have 10% leftover in profits. 

I know someone at GSU and much of the money allocated for other projects is being pushed over to their soon to be new campus, the old Braves Stadium, and there's a brewery and some retail scheduled for later this year on Georgia Ave.