From the Original Post:
I stumbled across what seems to be the best calculator for my purpose. It relies on wind, gradient and several other measures to compute a flat course equivalent. It highlights the relative importance of headwinds and maximum gradient in making a course difficult. For example, the Sherman pass (H99) climbs 1627 meters over a distance of 24.5 kms. According to the FLA calculator, these 24.5 kms are equivalent to 87 kms on a flat, windless course. The climbing effect is equivalent to headwinds blowing at 50 kms...
Interesting. It's got to be a complicated problem, since even flat roads usually have some small hills and may have headwinds. I generally use "200 feet of climbing = 1 extra mile" as a very rough estimate, but ignore it unless the climbs are well over 1000 feet high. That matches up fairly well with the calculator's blog page chart, showing a range between 1000 feet=3 miles for very strong riders, down to 1000 feet=almost 10 miles for weaker riders.
Difficult climbs:
If I have to stand up on the climb, that's going to make it a lot harder. With low enough gears, I can stay seated on an 8% grade quite easily (with no touring load!) 12% and over is going to be a strain for me. For really steep climbs, say 18% or more, I'd want to keep the day's mileage way down. (I can do steep or long miles, but not both at once.)