David,
I use ‘TimeSlice’. You can have it set to activate when you open an app, or start it manually. It is not tied to FCP, it will work with any app. It will track your time and allows you to assign rates to jobs, so you get a billing amount. It also lets you set a fixed budget, and work from that so you can see how much time/cash is left. I think it was about $30.
I use Quickbooks basic, but if I remember correctly Quickbooks Pro has a time tracker, that you can invoice directly from. You might want to look there too.
Scott Sheriff
Director
SST Digital Media
https://www.sstdigitalmedia.com