A recent update to the Rental Property Calculator appears to default the down payment to 100% and mortgage interest to 7%.

I own a home - my primary residence- in Seattle and plan on developing the back portion into couple/several units.  I will be able to subdivide the lot - back units will be condominiumized.   I will either sell all th...

Then they would be prepared to read term sheets and do basic investment calculations and would prevent them from doing something they dont understand. 

So be extra careful on properties owned over 3 or 4 years.

Structure Based on Total Project ContributionIf your land is free and clear, I’d calculate what percentage of the total project cost it represents.

Every extra dollar you put towards it will save you 6.375%, so it's basically a savings account with the same rate.If you have an emergency fund, need cash for other investments, and like to leverage debt more, I'd save all extra cash for the next investment and not pay extra on the principle. 

So, an extra $608 a month doesn't come close to smoothing that out.

No conventional lender will give you a second behind it as they will know it's not assumable.If you put 10% down a reasonable interest rate is 7.5% but it MUST be greater than the monthly APOR Average Prime Offer Rate or more than 4.5% or they get taxed extra.

Secondly, where did the 6 year payback calculation come in?

Payback period on Unit 2 rehabIf rehabbing Unit 2 costs, say, an extra $10–$15K, you’d make that back in ~18–24 months from the additional $600/month rent.