so should i mix 50% starter with 50% layer and add some eggs shells on the side.
would be suffice?
Maybe, maybe not. The reason it's hard to figure out is different birds have different calcium needs, and you won't know if it's enough until you start getting eggs and seeing the egg quality.
It'd be fine to try that at first but you may find that you still need to supplement more later on.
I feed both grower and layer (not mixed, but both are available for the hens to choose), with oyster shell on the side.