@EJ Hyman All 3 are great, but to accurately answer your question, it honestly depends on who you're looking for..If you're looking for off market deals - direct mail works rather well.

I know that doesn't take into account the finish/quality of the homes, but is that method generally accurate? 

Try walking into your bank and asking for a 30 year loan to buy some bitcoin lol.Personally feel like bitcoin is nothing more than gambling. 

Don’t know how accurate that is but it made me less concerned.

I'd like a better, more accurate figure, but it's a start. 

How accurate is it?

As far as loans for the purchase look into sight unseen loan programs for auctions Gives you 85% of the purchase with an adjustable rehab budget that covers 100% of the rehab upto 70% of the arv (Most of these properties you will not be able to get inside for an appraisal or to formulate an accurate construction budget) 

I am not a big fan of many of generic rules but the 50% rule (cost not including mortgage associated costs (PITI) is ~50% of rent) I believe to be semi accurate in low rent locales. 

Keep in mind that the cap rate will always be determined by the sales price so unless one is taking that into consideration while running numbers, it can be challenging to give an accurate cap rate for a given area or type of property.  

Seeking off market discounted properties in Houston Texas that have not been flooded. 3/2 or 4/2 brick construction price range for 50k to 200k with an accurate arv.