Well I do bid by job but when I come up with the price I consider my hourly rate say I think a job will take 4 hours I say 175 dollars for the job. You have to also consider difficulty off job if I had a 4 hour job with 25 foot arbs they want walled and formal I would probably be close to 300 as I will be up on a ladder moving around a very sharp piece of equipment in odd ways.
If I had a job with a bunch of shrubs you formal prune into globes super easy 175
Natural pruning takes much more time and few people can do it properly I would charge 250 all these are 4 hour half days jobs. Never tell the customer the hourly rate. If you bid a job 4 hours and it takes you 3 the customer will only want to pay for 3.