It is better to create a system to narrow the field of properties, and then run the math on the ones that fall into the narrowed criteria. It would also be hard to effectively make offers without doing the math first, so I think the method would be counter productive. Also, just like anything else the more time you spend qualify deals the better and faster you will become at it, so the practice will improve the process.
I would start narrowing by picking a house type in a specific area, specific price range that also fits certain criteria like number of bedrooms, etc... As you clear out these options start opening up your criteria a little and run the numbers on those houses. This will also help keep you on target to invest in the types of properties and areas that are your target market.