Shell grit and 12 - 17 wk pullets

It's possible (not counting my chickens etc) that I may be able to get some pullets tomorrow (at last!)  They will be Barnevelder, Light Sussex and Barred Rock breeds aged from 12 to 17 weeks.

I am aware that
a) Pre-lay pullets don't need shell grit
b) A high calcium diet in their early days can cause them problems in later life
c) Point of lay for these breeds may be later than others eg. 24 weeks

The feed I bought when I was young, naive and optimistic about getting chickens quickly (about 2 months ago) is a mix of whole and cracked seeds and grains, with bits of oyster shell mixed into it.  It's formulated for laying hens.

So my question is this: will the pullets self-regulate and only eat the bits they need, discarding or ignoring the shell?  Or am I putting their health at risk by making oyster shell available to them in their feed at this early stage?

I understand that many of you have crushed shell available in another container, but I'm wondering if having these bits of shell in their food container might encourage them to eat more of it than is good for them.

I have a pot of mixed grit & shell down for mine just in case them they can help them self if they need it

At last, Lindeggs - we're all relieved that the day has finally dawned......

Good news Lindeggs has her chickens