Now you have zero fate, can you still use fate from the luckstone?

Not sure what's the question here.

Obviously you can spend fate from the Luckstone, no matter if you have zero or more fate of your own. This can be either used for rerolls, or spent for effects that require spending fate (e.g. Nomad ability, or several effects in the Woodland expansion). The wording "Whenever you pay a fate, you may discard 1 fate from the Luckstone instead" does not suggest that this can be used only if you have at least 1 fate left.

Or are you asking if it counts as your fate for effects that consider your fate total, e.g. Verdandi or Titan Wraith? Then the answer is no, it doesn't count as your fate.

I'm not sure if this is needed either: but as far as I know you don't lose the fate on the luck stone if you for some reason have to discard all your fate.

Thanks I thought so just wanted others input "its good to get from time to time" .