for domestic, i try to workout how long it should take, then work from £30/hour.
so, 20 min = £10.
then I look at time/distance to get there from home. and charge £0.20/mile or £0.50/mile, depending on my mood

10 miles = £2.00 or £5.00
(I work each job as if it was the only job in that area)
cause remember when you quote you may be out the house 8hrs, but they are not all earning hours.