Actually around here, Grower is 16%. Grower/Developer, which is intended for age 13 weeks until you switch to layer, is 15% protein but it is not often available. If you are developing a layer flock you want to give their skeletal structure and organs a chance to mature before they start laying so high protein levels are not required.
If you are growing meat birds, you want a higher protein content. If you are growing chickens for meat and developing layers in the same flock, you can increase the protein some as a compromise. Flock Raiser or the combined Grower/Developer are good choices. It is not the best of both worlds, but for a mixed layer/meat flock it will work. If they are eating stuff that is not high in protein then the higher protein feed helps bring the protein level up where it should be. It is not really the percent of protein that is important but the total volume of protein they actually eat in a day.
With all that said, right now I am feeding 20% Grower/Starter. It is all that was available, other than Layer, when I went to
TSC yesterday. I'd prefer to feed them the lower protein levels, but I do not think I am hurting them by feeding higher protein levels. I will switch back when I can.
I don't see it as something to really stress out about. It is good to know the goals and what to aim for, but this is really a pretty wide target. Just use what is available to you and you have done the best you can.