Ooh, breed discussion! *bounces up and down in chair*
Slate legs are characteristic of australorps. If it had yellow legs it wouldn't be a lorp - people wouldn't be able to distinguish it from a BR.
I would just experiment for a bit and see what you get. OR if you know how the poultry calculator works, you could play with that and then be relatively sure of what you'd get.
I've tried the poultry calculator, it's wayyyy to confusing!