As of now, I'm just working backwards, figuring out from my agent what Sold prices per foot the area can withstand, designing a house with square footage to match the neighborhood, making sure there's a large enough margin between what my turnkey cost is and what I think it will sell for, and then finally splitting what the $ left between what I can offer for a lot and what I wanna keep for profit. 

Features such as number of bedrooms, bathrooms, square footage, etc. should be considered when making a determination.Local property managers can also be helpful.

I am considering purchasing a 1940's home that we will need to add sq. footage to and vault the ceiling.  

You can use recent sales prices (don't go back further than 6 months, but paying very close attention to the last 45-90 days), calculate the dollar per square foot to use for profiles within the same square footage range, and create an AVR for the subject property based on what it will offer after the rehab is complete. 

It's more like additional rooms, additional square footage like finishing a basement, adding things that would increase value like extra bathroom.

Two recent comps:Comp 1: Same model w same sq footage and floorplan sold 10-22-11 for $139k cash short sale via MLS. 43 days on market.

I pay about $300-$750 for something line that depending on overall square footage.

Beds, baths, sq footage and cost?

Your home might not be similar but it's all about square footage.

Just give them your criteria (area - county, city of zip code, building type, square footage, etc .etc.)