One of the 'mistakes' (conscious at the time) I made when doing technical consulting remotely was only billing for productive, focused hours when I'd be actively typing and mousing on the problem.
Someone suggested that, if I wanted to go for a walk to think about a problem (which is something I did), I should bill that. I decided that was a slippery slope.
Had I been working on-site, which consumed all my time without flexibility, then I'd bill for every hour on-site, and maybe for travel time.
But since we were doing remote (this was before Covid), with hours that I set -- and my clients were serious people, working on serious stuff -- I wanted to be serious too.