OK, so thanks to the earlier message answering to my question, masking from another layer works a treat! Plus if like me, your triggering from column's, you just need to set on the layer "ignore column trigger"
My only fear is as the show is triggered via column's, so if one was missing on that layer ("shouldn't happen!")
it wouldn't be like the mask being locked in on each layer.
I'll have a play with 50/50 and see how i get on...