But also you can more easily pull that money back out in the form of a HELOC much easier from a primary home compared to the rental.So in the end do you want peace of mind or mathematical optimization (which some would argue is leverage even more!)

I know they installed rear unit completely new roof in 2017, so the roof in front unit is the main concern for me that will need to be replacement in future, what I am thinking is that if I back out, I lose 11k, which the flat roof in front unit will cost less than 11k, I think you got great point here and I probably will stick with inspection in future, but mathematically speaking I was leaning towards account for roof replacement in future ?

So it is entirely possible that you could be required  to accept somebody with ZERO verifiable cash income,  for example being required to accept somebody who mathematically won't be able to pay all the rent  (say they have $1900 sec 8 voucher on $2000 rent with 3x income ratio,  and $300/mo SNAP benefit, and zero verifiable cash income)  If you get that situation,  contact an attorney before doing anything else)FWIW if you have reasonably strict criteria on prior landlord reference,  credit,  etc. 

They're directly/mathematically related - reciprocals.   

@Danny McGreevyPutting down a smaller amount and investing providing you have some hold time is mathematically better.

From a strictly mathematical standpoint, you have debt on the house at 4.8%.

If we rope the expense in with rent and assuming 20% down you’ll see appreciation gain of 20% year one reducing to 4% at year 30.You should build a better mathematical model before your purchase but at slightly negative on cash flow, I am expecting returns of 13% over the first 8 years with an 8.5% interest rate on the mortgage.

For those who might be mathematically challenged, let's break this down...You just said that a professional tenant could live in your unit for 18-36 months before you can evict them and it could cost "easily...10 times" the $15K price of training. 

But they were just mathematical speculations on the success of mortgage bonds as a whole.

It is a very complex mathematical system that we investors need to be keenly aware of.