Several years ago, like early 2000's ECM published an article that claimed the average profit for an electrical contractor the year before was 1.8%. I think that was accurate as it is. Like others have said or implied, profit is a moving target. If you drive a truck paid by the company, and your lunches are written off because you are with clients or on the road for business every day, and use other perfectly legal methods of reducing your tax liability, then that is money that is not "profit", but is a perk of owning your business. You really need to have a grasp of what your overhead is. Tools, shop, trucks, any loan interest, liability insurance, printers computers, electricity, everything. Then as a separate overhead, you need to know exactly what a manhour costs. This should be kept separate from general overhead and include, unemployment insurance, workman's comp, vacation, holidays, training and safety meeting time, personal safety equipment, union or association dues, etc. Then you have to use a labor rate that covers the second one. Now, for a hard bid, it is virtually impossible to know the exact number of manhours you will use, so that has to be accounted for unless you are working by the hour.
From that, my feeling is every job should cover its portion of overhead, this is difficult because many overhead costs are dollar amounts that change little or none in ratio to the amount of work in a year. Basically overhead is likely to be 20% of you total gross for the year. Your profit needs to be on top of this. So the 30% that was bantered around is not profit, it is overhead and profit.
So for hard bid pricing profit and overhead are a moving target. For a time and material cost, a decent rule of thumb is employee actual pay rate, time 45% for overhead (above) and then doubled at a minimum or more if the area allows because you will likely only bill 5-6 hours a day for an 8 hour employee. Then 30% on material, unless there is a big ticket item.