I usually charge $2/inch up to 48", then I go to $3/inch. That's for a few stumps.
On larger jobs, if the majority are of similar size, I'll usually bid a per stump price. I base it upon how long I think it'll take me to do them, & trying to hit my target range of $100/hour.
I bid one the other day, 27 stumps, mostly larger. I figured out the average size (33") & priced it at $2/inch ($66). Came up to just under $1800 total. I bid $1600 to do the entire job. Haven't heard back, but no one else has done the job yet.
All jobs are different, but I try to be as consistent as I can. This is a part time gig for me. If I'm makin' $100/hour for me & my machine, then I'm doin' better than if I worked OT at my real job. Sometimes at the end of the day, I feel like I've accomplished that, & other days, well, I fall back on the "grindin' stumps sure is fun" attitude.