Reflaunching Chimney Crown - Cement thickness

[tododo]

Hi all

We have just repointed chimney we and now doing the crown - it wasnt in too bad but there were some loose bits that needed removing and some roots too

We cant remove the crown altogether as its a bigger job than we realised when we started so we are going to recover with a layer of cement

What sort of thinkness should this new layer be?
I think I have cement fibres here - worth putting them in do you think??

Thanks