An informal rule of thumb I've found is that if two months rent is greater than property tax and insurance combined, the numbers usually work well.A couple random expenses I've had from my two properties since April (after tenant moved in):Central air issue, broken screen on screen door, lack of hot water in shower, clogged drain from toiletI'd be careful in renting to a friend. 

As for the water/sewer it is in good shape.

Then she stopped paying the water bill in full. 

Lot with city water/sewer, house was torn down.

Typically a duplex will offer twice as many problems that could arise. 2 kitchens, 2 bathrooms, 2 water heaters, 2 furnaces and 2 tenants.

Water heater is not installed, (It is just sitting on the kitchen floor)2.

I mean changing furnace filters, water filters, yard/lawn maintenance etc.

The end goal is to have have to have enough parking for a single vehicle per unit, to separate the water and electricity, and to convert the single family into a duplex. 

It is a SFH so I do not have 24 months of utility records but the average is around $450/month for power and $100/month for water. 

If you bumped each of those up to 10% as well, I wouldn't object.I'm also curious whether you're sure about the electricity, water + sewer, garbage, etc.