If you think the value will likely be raised by 25% after you buy, I'd multiply the current rate by .25 to get in the ballpark #s. 

The earned finance charge is computed by multiplying the daily rate by the number of days the principal balance is outstanding."

If you multiply that by 24 months or 2 years, you're saving $5,418.96 so you'll recoup your money in about 20 months which is good.

First you have to figure out what the grm (gross rent multiplier) is for each market area.

So if cash is what you are looking for to purchase more RE, you need to know the equity of each property and multiply it by 65-70% minus current mortgages and that is what you can get out of each property minus closing costs.

I've been looking at the Bigger Pockets BRRRR calculator and I think the "gross rent multiplier" is being calculated incorrectly.

More specifically, at 20 units and they each rent for $1500/mo, that is $30,000 total monthly and multiplied by 12 equals $360,000/yr gross rent.  

Then multiply by .25 to approximate the increase in interest rate vs paying the discount in points?

I usually find about 5 similar comps & divide each of their sold price by its total sqft, get the average price per sqft of the comps then finally multiply the average price per sqft of the 5 comps to the subject property.

The tax assessed value for that property is $105,540 multiplied by the unexempt tax rate in Dallas of ~2.74% is $2,885.