Hi there, hope you are enjoying BYC!
Don't do it, they're way too young and feeding excess calcium long term to birds not in lay like juveniles, broody's, roosters, molting, etc can stunt growth, delay the onset of lay, cause gout and kidney issues including failure and death.. There is likely a genetic factor since not every bird will experience that, but it is a known concern. Stick with starter or grower until you see your first egg. Oyster shell can be made available
on the side as you get closer. Please don't be fear mongered by this statement or by the worry of a soft egg.
New layer hiccups that can result in soft eggshells are caused by the system coming online and working out it's kinks.. not by lack of calcium. Don't diminish your protein content this soon as nutrition does effect ALL things health and immunity wise.
With daylight diminishing.. you're at least 6 weeks away before your first egg.. and likely longer than that depending on your breeds and lighting conditions. .
Welcome to BYC!
Make the OS available free choice all the time. Layer should technically have enough calcium but some heavy layers may require more.. my birds never used the OS back when that was my feed routine. Now, I serve oyster shell on the side of "flock raiser" full time (in addition to their egg shells).. and the difference in my flock is noticeable.. due to the higher level of protein but importantly amino acids.
Ever had a fecal float or done any worming?
You say mature hens.. how mature? with age.. egg quality diminishes.
These things can also be caused by hot weather or approaching molt.