You still gotta go by time and materials. I've made a list of trees tha were .5, 1, 2 & 3 hrs to Complete. Tick marks in rows to get a time estimate.
I find that the brush and vines are the worst part, most trees will go through the chipper without a hitch. With the 18 inch chippers you can bunch several small trees and just stuff them in.
Another time saver is a dumpster for all the trash logs that can't be chipped.
Once you've done an few you can get a feel and bid by the acre. Be aware this is another arena for the lowball freaks.
Large jobs get ernest money and a payment schedule so that you do a part,get a check, then do another part. Have legal councel write a contract.