I won't post it again, but just a quick note; remember the 70% rule is a rule of thumb. It's not useless like the 2% rule. But don't take it as Gospel. On more expensive properties, you can settle for less, on really cheap properties, you'll need more. Sq. ft. for sq. ft. a roof costs the same, and you'll have less margin for mistakes and overages on cheap properties.
Read the Ultimate Beginner's Guide. If you don't understand the 70% concept, I'm sure there are plenty of additional, very important concepts you're missing.