Same way you would calculate it in normal investing - once the property is 5+ units, it gets a little more nuanced.

My dad considered a six month to one year vacancy between tenants normal.

Always be in front of the contract and prioritize where you're buying and entry price. lf all those are three in your favor, you have a lot of leeway with some of the normal but difficult parts of a fix & flop/hold.

They are in Fountain Square, Crooked Creek, and Forest Manor/Audubon Gardens.I normally see expense ratios sitting around 40-50% for SFH's, is this accurate for this area? 

Purchase price: $75,000 Cash invested: $21,400 3700 SQUARE FOOT DUPLEX. 4/1.5 AND 4/2.

Is it normal to have a negative cash flow in the beginning?

People like to fight on me on this and there can be exceptions, but for a ton of different reasons, they dont normally make great investments. 

I think these bigger cabins are getting sucked up by the normal demand and with the lack of new builds coming online. 

Unfortunately very few who seek out these assets take the time to consider their exit and fail to realize they are the most costly assets to exit, let me explain why: Here's how that disposition process to an owner occupant looks: -You will incur turn over expenses associated with vacating the tenant in preparation of the sale -The natural buyer in this price point is the FHA buyer who will normally seek a 5-6% seller assist  -Passing a Section 8 inspection/preparing a home for a low income tenant is very different than the expectations of a home owner,   particularly the FHA buyer who cannot afford to take on any deferred maintenance.

I know a HELOC normally does not come with much of a closing cost, but over the long-term, this might make the most sense.