Then you can multiply that by the ADU gla and have that as an idea. 

There are several considerations as @David M. addressed, but a rule of thumb is to consider your Federal marginal tax bracket as well as your applicable state tax rate and multiply that by the deduction to estimate your tax savings.  

Now multiply the number if piers by $250. 29x250 = $7,250.Now add $200 for permit & $400 for Engineer's letter (yes, both of these are required by the City). 200+400 = $600Total foundation price = $7,850.

However, Using the gross monthly net per property, with a multiplier of 1.8 and then multiplying by 12,  I can’t fathom this $675,000 number.  

When he entered the gross monthly income to the rental property gross monthly income, he multiplied the $725 by 4 which showed grossly month income of $2,900.

It wasn't meant as literal 20 doors exactly, but more an expression of wanting to multiply what I started with.But I get where you're coming from in that last paragraph.

The amount of work and stress will multiply so I would just be cautious about that

You can use monthly periods and the resulting IRR equation will deliver a monthly IRR which you can simply multiply by 12 to find the annual IRR.  

For MTR - look up the same property for LTR rates and  multiply it by 30%.

So you'd be multiplying $20, 000 by 20% to get $4,000.