My real estate education came from "Hard Knocks Approach."   

I've mailed out letters to homeowners in pre-foreclosure, bought "We Buy Houses signs, and I've even knocked on doors.

In this market it needs to knocked out of the park.

Roof costs say $8000 (depending on size, shape, type of shingles, etc.) and last 25 years (depending on weather, type of shingles, slope, etc.) ... that's $27/mo you need to put away in CapEx to eventually replace the roof ... now do that for all the systems in the property, add them all up, add some reasonable amount of margin, and you have a reasonable grass roots estimate of what your CapEx should be ... code it into excel with the approriate parameters such as square footage, number of water heaters, etc. and you can knock out a CapEx estimate for a specific property in a matter of minutes, without messing around with bogus rules of thumb like 50% rule or 10% of gross rent, or any other such thing that tries to relate things that are completely unrelated in order to shortcut around actual thought and analysis.Any calculator that you do not understand all of the inputs, outputs and assumptions that went into them are less than useless, and BPs are no exception ... therefore the best ones will be the ones you build yourself from the ground up and modify over time as you gain experience and find that some of your assumptions, inputs, and formulas need tweaking.  

You might try a few Dave Ramsey podcasts for ideas, steps and motivation on getting some personal debts knocked down ; he also likes rentals but favors paying all cash for rentals (which is great but may not be reasonable for many folks, especially on the coasts and costly markets).

Contacting failed listings, also know as expired listings is another great way to do transactions - but forget the letter - go knock on the door - ask "are you still accepting offers on this property?"

They can knock it out really quick and they seem to do good work.

If this is a concern, go knock on the doors of the neighbors in that neighborhood, explain who you are and what you are looking purchase, and ask them if they would be willing to share their experiences.  

I'd love to go door knocking knowing they were behind a little.

But I know I wasn't thinking that at 17, so it's hard to knock you.