^^^ Totally agree OldChippy.
Most builders I know are charging 10-20% on top at the most.
When I'd work out a quote like you I used to charge what I could buy materials for, work out the labour cost and add 10%.
I still used to miss out on some jobs by a big margin.
I was lucky in that I ended up with a good sparky and plumber who were very reasonable in their costs.
Stewie