I think this shows the fixation on hourly rates. Why does there need to be a translation to hours.
A day is a day. When I was freelance I just billed a day rate. The point for me is that I simply don't count the hours. It just meant I would spend the day working on that clients project only. Typically I'd do something like around 9-17:30, with around an hour for lunch. So that works at 7h30. But sometimes I stopped earlier. Sometimes I worked later. I still billed the same.
I never pro-rated the invoices for partial days. My minimum billing increment was half a day on the odd occasion I needed to run an errand that was going to require less than a full day off.
That is what I do. Have a daily rate, but can bill half a day.
I don't really do classic 'projects', I'm more an ops guy having seen/experienced the dev side too, and profiled myself as being a bridge between those 2 sides that don't always fully understand each other. At a client I'm usually involved in multiple projects and 'the bigger picture'.
You cannot place an estimate on 'fixing' stuff like that, and it also hugely depends on the internal politics. My contracts are usually for 3-months, and can be extended 3 months at a time, which define a daily rate, state a day is 8 hours, and I can bill a maximum of 40 hours/week. Contractually I cannot terminate the contract, only the client can - but I can simply decide to stop working for a client, and simply don't bill them until the contract expires.
This allows me the flexibility to work for multiple clients, bill half days, work when I want, and can stop working for someone if I decide it's not working out for some reason.
You get my productive output for normal working hours, typically an 8-9 hour range, but I'm not tracking time spent on potty breaks, eating lunch, or taking a walk outside.
