Overhead costs typically are incremental and remain pretty constant (unless you have one property per LLC), 1 property with $3000/yr OH costs doesn't mean 2 properties will cost $6,000/yr, unlike operating expenses which are usually multiplied by however many properties you have.

Also, you will need calculate cap rate off those numbers, cash-on-cash return, Pro forma, Gross Rent Multiplier, and Debt Coverage ratio.

currently in the process of applying for mortgage Staten Island NY 10308Property cost$450,000 $90,000 down payment (20% down) $360,000 mortgage 3.7% interestClosing Cost$20,000 Construction Cost$150,000 Built in $10,000 contingency fund for construction expensesHandeling cost25,000$2,175 monthly mortgage payment multiplied by 12 monthsTotal Investment$645,000 New Project $275/square foot$855,000 $275X3100 Sqft projected sale priceBroker commision for selling$45,000 5% broker fee$825,000 $825,000 projected sale proceed minus broker commissionTotal investment$645,000 Total investmentprofits after everything$180,000 Net profit

Once you’ve established the ARV, multiply it by 70%, and then subtract the anticipated cost for repairs. 

You need to factor in the penalties for paying this off early as well as the interest you'd be paying until you pay it off.Basically though, you're saying that you'd build each townhome for as low as $132,000 then sell it a few months later for $300,000, except you're multiplying it by 143 units.

I'm trying to get my head around cap rates and gross rent multipliers. 

You can go here: http://www.lubbockcad.org/Appraisal/PublicAccess/ and do a property address search(s) to see what the current appraised valuation is set at and how much the tax bill was for at least the last 5 years.Then to be safe, take the sales price (likely larger than the appraised value, but not always) and multiply it by the tax rate listed on the bill for that property.  

(think of the worst car salesman stereotypes and multiply by 100x and throw in some illegal sales techniques, that's finance) lol. 

As far as age, I have been using a multiplier if it is more than a 10-15 year difference.

I am not paying 8-10% management fees of any of my $1,200 - $1,500 rents... multiply that by just the 12 sfr's we own and manage and it does change the formula, doesn't it?