Charge by the hour ?

I've shared this before in here but I'd highly recommend doing block pricing hybrid to the typical hourly if you need some sort of time and materials pricing. Half day and full day. For a couple of reasons - way easier to administrate. ideally you'd want to get them to agree to some sort of price ahead of time. Sure you want to be protected (which is why anyone does hourly) but you also want things to be easy to administrate on the back end. One thing we ran in to when we did hourly was people saying that they had a 15 minute convo about future work that shouldn't be paid for. Basically being hourly invites all the cheap people just because you don't want to think about the job on the front end. 

if you work 4 hours somewhere, you can realistically go to another job and work 4 hours. if you work 6, probably not but then you missed out on two hours of potential pay but you reserved your day for that person. Long term, that's bad business. Also, the more you chop up a price, the more likely someone is to run you around so they can pay as little as possible. We used to do 1/4 days, 1/2 days and full days. Even thought he half days are more, people would still complain about the 1/4 days then run my guys around like crazy then we'd have call backs. Now we do half day minimums and we don't have to drive across town to change one doorknob and make $100 as a company. 

Hourly seems safe but long term it will hurt you. You still need the ability to do something similar so break it up in to half day blocks. I know guys that ONLY charge full days. I'm not there yet but I like that. Trust me, you're always going to have price resistance. Just have price resistance and also make money. That's way better. 

Even better than that, have fixed prices for things. You can quote people faster and take advantage of being efficient.