I'm not commenting on your maths as I havn't checked them.
However you do make valid points about the time spent 'cleaning' and everything inbetween.
If it helps an accountant, for examble, wont spend 36 hours a week 'working'. Neither will any other office worker. Think of the lost hours going to the loo, talking to the collegues, daydreaming and the like.
Many offices like to up productivity but then they become sweat shops.
For us, all the customer sees is the time when they eventually clap eyes on us till the time we hold our hand out for payment. almost 100% time 'actually working'. There are of course other things to do like having a pee and daydreaming!
I'm sure some people look at the profit that car salesmen take in their half hour with the customer and think WHAT! Say a 2k profit for less than an hour. But thats not a fair reflection at all.
When customers comment about the time/price ratio I sound shocked and say "Oh dear is as long as that, I usually aim for £xx.xx" then double their number.
Our hourly 'on the job rate' should be way higher than our turnover divided by 36 hrs, which will be way highr than our actual spendable income.