There should be a label either ON the bag, or attached to it, that gives the protein level, and the ingredients. By law I think there has to be one...it's possible that it was ripped off the bag, tho. (sometimes it will say "meat products" or "plant protein" as a catchall for whatever they use that batch....some ingredients are hard to get month after month in all areas)
The chick starter has minerals and salts in it that the chick needs to grow well. There is a product called "poultry nutrients" that a farm store can usually order for you if you really want to mix your own.
from what you listed, you gave them maybe 10% protein, which is WAY to low. Although the milk powder, depending on how much and what type, would raise that. chicks like to have as much protein as practical....18%being the norm for a "chick starter". Some of us use 21% protein, or higher.
Birds out on their own foraging will eat a lot of insects, worms, etc. to get the protein. Also weeds like plantain, dandelions, etc., as well as whatever seeds drop to the ground.
is an excellent place to start if you're interested in what is fed to chickens, protein levels, and "the ideal feed".
eta: every animal needs salt to live. if you continue to mix your own feed, and don't get a poultry nutrient, you might want to add something like VitAl or Poultry electrolytes to their water.