There must be other ratios that come into play to limit investment purchase size to income? 

You need to check for setbacks, frontage, lot size, elevations, and flood zones.

And since you are looking to build legacy, typically that comes from building a buy and hold portfolio.I don't mean to assume that find land, getting it entitled as needed, and then building a property of any size is "easy", but with the added work often comes the potential for better returns, both in cash flow and value.  

And we know there is NOT rampant "bloat" in rent prices because if there were, it's also simple math that landlords would be rushing to create additional units to capitalize in that "pay day" right, because it's "bloated" rents, meaning over sized profit margins, which to be true would mean it's "cheap" to create new units vs the over sized profits it's producing.......

I found it really time consuming to measure for every single repair in a house so automated calcs as much as I could based on room sizes.  

Since Section 8 has a particular amount they allow for size of unit you should check that the amount for your unit size.

So if I were in your shoes I would want to be really confident in the exit strategy. 1) confirm rent estimates 2) confirm ARV and the demand in that area to buy a house like that in the price range 3) are there banks willing to refinance at the LTV you want with the seasoning period you have. 40k for a house of that size that needs that work would be the cost of materials alone where we live in WI, so verify your estimates.

Purchase price: $400,000 Cash invested: $57,000 Type: Single Family ResidenceParking: Garage - AttachedStories: 2Bath: 3 Full BathsBuilding Size: 2,686 Sqft.Lot Size: 50,791 Sqft.Year Built: 2005

Rents are of the charts.

I cannot see how build two, say, 1200 sq ft houses with two roofs, two foundations, presumable about 25-30% more windows and siding is going to be cheaper a duplex that might have a little extra fireblocking and soundproofing.Now, maybe your GC is thinking that detached homes tend to get higher rent than comparably sized and finished duplexes.