Here's a couple links that will take all the "maybe this - maybe that" guessing out of the equation...both examples below were suggested to me quite some time ago by some very experienced pilots at one of the clubs I fly at......neither examples have led me astray and take a laymen's approach to the subject....
http://www.rmac.co.za/Rand%20Model%2...e%20Basics.pdf
http://www.themarcs.org/index.php?pa...wer-your-plane
Hope this helps those in need.....as well as those who still rely on weight as the key ingredient for all set-ups.....!