The standard for step ladders is to measure the length of the ladder when folded for storage. Typically, the top "safe" step is about 2.5' below that. So, for a 10' ladder, you're down 2 steps (the top and the last rung are not safe) which is about 2' and then another half foot due to the angle of the ladder when deployed, so the highest safe step is 7.5'. Then for "reach" they add about 6.5' so you can reach 14' with a 10' ladder. I'm not sure on extension ladders, but I assume it's pretty similar.
Got my Louisville FXS1412HD today. Pretty good from Home Depot - ordered on Sunday arrived Wed morning. I climbed it today and it's quite stable compared to other 12' ladders I've climbed. I did install the LadderRail and will try that out tomorrow to see if I can reach the skylights to clean/change out the screens. I think rather than try to sell my Little Giant M26, I'll just deploy it as an extension ladder on the side of one of my water tanks, replacing a redwood ladder I built over a decade ago. It's overkill for that, but I suspect selling my M26 probably won't net me much.
The Louisville I just got is their "Cross-Step" design, which is OSHA approved to be used as a leaning ladder due to the rear legs being hinged differently, the top piece having rubber to increase friction, and the bottom feet designed slightly differently. As a regular step ladder, it gets you closer over the top, which is cool, and reduces the spread, which for a big ladder like mine is really helpful indoors.