The type of work, and the resulting type and quantity of material, surely does make a difference here, especially if you are only going to charge for labor on the job site.
If you are charging for a handful of receptacles that you had to spend an hour on going to the store to obtain then a 300% markup on material is justifiable, or else add an hour to the labor section of the invoivce.
If the material is $5000 of wire that also took you an hour to pick up from the supply house, then marking up the material 200% is not justified.