You guys are confusing the hell out of me and I have a Master's Degree. There is no simple math that can be used for every unique situation. For example, every post mentions this 4:1 ratio which the average reader would assume is accepted and safe for every roof height, or that the 4:1 ratio can be achieved at any house. In my situation, I have 2 issues with the 4:1 ratio. First, I don't feel SAFE with a 4:1 ratio (75 degree tilt), and my max roof height is only 18 feet, but I do not plan on actually getting on the roof because of the severe pitch. Second, due to numerous obstructions around my house (bushes, overhangs, etc), my "math" will force me to purchase a ladder that has more like a 6:1 ratio, which will account for the obstructions and simultaneously provide a safer tilt.
So here is my math for the group:
Step 1: Formula AH (Actual Height) + DO (Desired Overhang) + DP (Desired Pitch)
Measure the actual height of your roof line (18 ft in my case), plus desired overhang (in my case 2 ft will do since I am just cleaning gutters and will never get on my roof, plus desired pitch that you choose for safety and obstructions (6 ft from house in my case). 18+2+6=26
Step 2: Find a ladder with an actual fully extended length of 26 feet or greater (regardless of what the length is advertised as).
If you math wizards find something wrong with my math please let me know ASAP, because I am going to buy The Gorilla 26 (with wheels) or a 28 foot extension ladder today or tomorrow.