I've read several different places that $30-35/sqft is a good starting point for a gut job flip and $20/sqft is a good approximation for a moderate flip where some systems (roof or ac) are ok, so depending on condition, I'll multiply either $35 or $20 x (2/1 house sqft) and then add $50,000. 

(they measured the footprint of the buildings, determined roof square feet and then multiplied x the "going rate" to arrive at a price. 

You're creating inventory that multiplies NOI. 

The taxing authority will have all of the various taxes published on their website(such as county, hospital, school districts and many others), you just add them up for the ones what apply to that property and multiply it by the sale price.

The cost per square foot to develop shouldn't change that much from a single family for a duplex... you just have to look at what the cost per square foot is and multiply it by how many square feet you're building. 

Can someone explain how the Gross Rent Multiplier works?

All items entered into the BRRRR Calculator as expenses (water/sewage, taxes, insurance) will automatically be multiplied times the Rehab period and transferred to the Rehab budget.  

@Joe Farruggio Multiply what it will be worth *.8 (80 percent).

At a minimum you'd probably be looking at 4% (multiply annual rent by 25).

Returns, cap rates and gross rent multipliers are several ways that people begin to look at markets.