The whole thing in my opinion is more of an art then a science. There are too many variables to conquer that aren't even acknowledged by traditional teachings much less what they are still trying to put together formally now in this area. I found real world test to be the most straight forward way to learn this stuff, it's about starting with the power/weight and scaling it all correctly from the drawing board. There are multi thresholds in flight characteristics within the scale/design/conditions planes fly through, most are not even known exactly and these are what throw off any theory or "system", you are trying to get an even curve in your data, but should be looking at a stock market symbol when it all comes down to it.