it would also depend on the pungency of the powder.
Half a teaspoon for the chicken sounds about right. It may get lost in the rice, though. I would suggest using a couple of fresh bay leaves, which contain the same flavour as green cardamom (eucalyptol, or 1,8-cineol), and release their flavour over time, whereas the powdered cardamom may lose its flavour very quickly.
As chicken tikka masala is a made up dish, there isn't a traditional recipe, so anything goes, really.
Note, in India, bay leaves are not used, but tej patta, which is the leaf from a species of cassia tree.
take time to smell the roses 8-)