i know this is an old thread but i figure why not answer anyway. all things considered I've found it helpful to have a minimum charge that covers most easy jobs. add per linear foot, and add for other things like screens and extra tall ladder. it helps not to have make a special trip to quote. consider discounts for regular cleanings. don't charge by the hour.
there are several threads that discuss pricing gutters and there would probably be a better response if this was posted on the premium section ... not many people discuss pricing, especially on this side!!!
Like other cleaning pricing, the local market generally adjusts itself. Set an hourly $ target and compare it to the production rate factoring in the variation you mention above to determine your cost per linear foot.