If your property is in tough shape or is older; I would increase this reserve, but this is a rough calculation for a property that is in good condition, not in need of major capex.You can also increase this annually or do it on a monthly basis; i.e. $25 per unit, per month goes into a savings account and I will increase that x% per year. 

The property was in great condition and 100% performing.

@Christopher Rodriguez depending on where in San Diego, size and condition of the duplex that could be an amazing deal or a mediocre one .

The home is in great condition and is worth $500k easily, state of Alabama.

How old and in what condition are the big-ticket features (roof, windows, doors, flooring, furnace, water heater, etc.)? 

That was because of the conditions of that specific market.

You can evict if certain conditions are met.

If your friend's work is limited from her condition that alone might allow her to apply for SSI....which is very stable income.  

Can you rent a property in the current condition for $2500 ? 

This may include, but is not limited to, property type (duplex vs. tri/quadplex vs. commercial), price range (max price), location (zip code), condition (Class A-D) and profitability.Good luck!