For an average rental, the window controls I provided a link to are FAR faster (which also means cheaper), easier, and foolproof.

While I agree there is no fool-proof way to determine rehab costs or perfectly accurate ARV, I can say with confidence that (for example) a fully rehabbed 3/2/2 with 1200 sf is invariably comparable to the fully rehabbed 3/2/2/1200sf house in the same neighborhood and NOT with the 4/2.5/2/1750sf house. 

Here's a near foolproof plan. 

It's not foolproof (and nothing is) but if another investor in your market has said "I use company X consistently" and then another investor says "I've used company X too and would recommend them"...then that usually puts us in a good starting position.Have you been to any local real estate groups yet? 

Every solution has it's drawbacks and nothing is foolproof, pick your own poison I guess :) 

It's not foolproof...but it helps.  

Fool proof!

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It's not foolproof but if another investor has used a company and has had good experience then that usually get's us in a good starting place.

Nothing is foolproof of course, but if someone else has experience with them and feels good enough to recommend them because they have vetted that vendor - then that's a good place to start.Visit some local real estate meetup groups.