I can confidently use a vacancy factor of 5%.  

Depends on many factors, needs of a seller, your needs, type of property, property condition, use of the property, potential of the property, value that may be added and time frame.Many other factors too.RE is not a step by step, deal thing, it is as much an art as a science you won't paint a master piece with a paint by numbers set!

Just make sure you run all your numbers factoring the increases cost of FHA loan payments.

I'm also noticing that when I factor taxes into the equation, it makes much more financial sense to buy-fix-rent-hold for as long as the underlying asset (the home) continues to appreciate in price at a rate that is at least commensurate with inflation, prior to divestiture.  

It depends on how long you have owned it and if you have made any capital improvements as well as other factors but a very "ballpark" number would be about $5K. 

But rentals do have their downsides - big capex, bad tenants, and local market factors can all affect returns in an unexpected way.

Cash Flow, Income and Expense, Commercial Enhancements and various complex equity waterfalls and many factors requires reasoned professionals to "size" up.

Those two factors can make the process challenging.

Take all these factors into consideration before acquiring the property.  

The numbers look good on paper, but the hassle factor is a very real consideration.