Some guys who go it alone that i know of say a minimum of 20% and wouldn't have it any other way. Doesn't matter how small a business, got to make a profit at every opportunity as long as it's not blatant robbery.
Tescos and all the other big companies put a ridiculous mark up on some of their their products, they don't make millions upon millions of pounds by accident. When it comes to it I will be putting a healthy mark up on everything, if a customer has any problem I will tell them they are more than welcome to get stuff themselves but if anything is wrong for the job or stuff is missing I will charge for my time either getting it myself or waiting for them to get it.