You should really be looking at the thread about mark up on this forum.
As I stated there, unless you know what your overhead is, there is no way to come up with a realistic markup. It would be very easy to use too low of a percentage & lose money or be too high & lose work. Talk to your accountant, he can help you get a handle on this.
As far as blanket markup is concerned, if you get the labor and material markup right & you do a correct takeoff, you don't need a blanket markup. This markup just looks like another place to make a mistake.
Once you start bidding a lot of jobs, you'll learn who your competition is & you should be able to get a good feel for how they bid their jobs. Make sure that you always find out the outcome of any bids you submit. That way you can compare your pricing to others. There is no reason for a GC not to give you this info after he awards the sub-contract. If he doesn't want to give out this info, don't bother biddiing his work.
Few things are harder to put up with than the annoyance of a good example.