In terms of arriving at what that number should be:  (1) take the PITI and divide that in half.

That is the current conventional loan limit for 2 family, which is $543K, divided by 75%.

Ryan,Best advice if you are worried: create an operating agreement between you and your dad that specifies who does what, and how the money gets handled/divided. 

Hello,This is my first post on BiggerPockets. I have heard great things about this site and I'm excited to get more involved. I am looking for my first deal and had an idea that I wanted to get some advice on. I'm ...

Purchase: For a purchase money transaction, the LTV is determined by dividing the loan amount by the lesser of the “as completed” appraised value of the property or the sum of the purchase price of the property and the total rehabilitation costs.Refinance Transactions: For a refinance transaction, the LTV is determined by dividing the original loan amount by the “as completed” appraised value of the property.Eligible Renovation: There are no required improvements or restrictions on the types of repairs allowed.

If we take P&I and multiply by 12 payments then divide that number by our investment we will get yearly ROI?

Then the net should be divided between both partners.

A separate LLC, how ownership is divided, the LLC own checking account. 

@Kevin Martin Well, no, what the OP said was "ALL IN", this means that on the construction side, it includes the land, dividing/re-platting the lot(s), plans, permit, utilities, government fees, building construction costs, flatwork, landscape, and occupancy permit, on the contractor side, it shall also include any and all profit, overhead, and any other contingency, on the investor side, it also includes the research, buying and selling fees, holding costs, agent fees, and any other expenses associated with buying and selling.

Or anything to further that in the design (be it assigned parking, more private entrances, etc) could help.Or another (cheaper) angle is to somehow make the marketing scream triplex/divided home.