It's better to calculate in hourly turnover, as houses come in different sizes. Set yourself an hourly target (£30 is good as a starting point) then when you price your jobs, think to yourself that 20 minutes is £10. Work out roughly how long a particular job will take, and price accordingly.
You should be able to do a standard 3-bed semi with no conservatory in 20 minutes.
I've used this formula for years, and quickly got to the stage where after (say) four hours I would be within five minutes of my target without ever having to look at my watch.