Assumed that the amino acids found in the protein are balanced, something of a minimum requirement for layers would be satisfied by a 12.5% to 18% protein feed, depending on how much the layers are eating. The more they are eating, the lower the protein content of their ration can be. Nutrient Requirements of Poultry
With the warm weather coming on, your hens may not be eating as much so the higher protein levels are what the animal nutritionists recommend.
If you are diluting their diet with low-protein food from other sources, the hens would also benefit from commercial feed with higher protein percentages.
15-18% may be OK, but was not enough for my birds. I had to switch them to Flock Raiser, which is 22%, to get them to stop picking feathers. I just keep a small dish full of oyster shell next to the feeder, so they can get it if they need it.