Well, I have formulas for linear footage trimming and edging...
But I don't have formulas per say for mowing banks and such with a trimmer.
I do enough of this work that I just figure the mowing, trimming and edging, then throw on an extra weekly amount for mowing with the trimmer. In other words I know about how long such a place will take me and I charge through the nose for it. I go $1-$2 per minute depending on volume and site difficulty... and I guess some of that comes from the fact that "I" am doing it personally.