If you are going to supplement calcium, and if you feed a modern compound feed pullets (1st year hens) shouldn't need to be supplemented, then use crushed oyster shell ad lib not added to their feed in preference to limestone flour. It's a well established fact from researchers that young commercial layers require 3.5% calcium in their feed (we're talking about 90% laying efficient birds, pure breeds and less productive birds need less). Compound feed contains 4% calcium in the form of limestone flour and oyster shell (usually 1% oystershell and 3% limestone). Older commercial birds usually refered to as 2nd and 3rd phase (38 to 80 wks) may benefit from a higher proportion (25%) of their calcium as coarser oystershell. The reason being they don't absorb calcium as well as young birds and it's thought that oystershell with a larger particle size remains in their gizzards and is used as a grinding agent for their feed so providing a slower release of calcium particularly for early morning layers. Even so as I said compound feed contains 4% calcium which is well in excess of even high yeild layers in normal health needs under any management system, including free range, if not heavily fed on scratch feeds.
This is what BOCM Paul (one of the leading UK feed producers) say about Farmgate compound layers feed
"A highly palatable feed specially formulated to meet the need for a general, all-purpose diet which can be fed to flocks of mixed ages under any management system.
Feeding GuideOffer on an ad-lib basis from point of lay. Farmgate Layers feeds do not require any additional limestone or oyster shell grit. The diet is fully balanced for optimum egg production, consistent shell quality, good yolk colour and excellent egg size."
All other manufacturers I've reviewed say something similar or describe their feed as a complete feed for any management system meaning for normal healthy birds there's they are formulated to provide all their nutritional needs.
Hence if you supplement do so on an ad libitum basis and don't force feed unless there's some medical issue and then you should also consider Vit D3 deficiency and phospherous deficiency not just calcium. Increasing Ca without balancing these others won't guarentee greater absorption.
I make oystershell available to my birds from 40 weeks or so but this timing is not critical.
Increasing calcium intake unnecessarily has no benefit and potentially can cause kidney problems as well as problems with other vital organs. see
http://www.poultryindustrycouncil.ca/diseasefacts/calciumandvitaminD3problemsinlayingbirds.pdfHF