You are a charity, or other worthy cause, or someone I really want to work with - 50% - 100% discount.

You are a normal client - 100%.

You are either an arsehole or have asked for something very last minute - 150%-200% depending on the mood I'm in.

The last one, colloquially known as the fuck-off rate, is probably the best bit of contracting advice I ever got from a colleague. He hated the gig we were doing, utterly despised the boss, and walked away smiling every day because he was one week closer to retirement. It's only really sustainable for your mental health in short runs.

Oh, and never charge per hour. It isn't like you can do an hour for client X and then the next one on client Y. You're not a bloody lawyer - you're a professional!

I sometimes split jobs into half-days, if the client prefers it. Although that's more like 66% for each half.

Oh, and finally, you don't customise Salesforce. You help your clients solve a problem. That may usually involve Salesforce, but most of your potential clients probably don't know what SF can do. So what you're actually selling is a way to optimise the client funnel (or whatever).