Mortar Mix Formula?

I hired a stone mason to finish a fireplace with "dry stack" archectural stone. I made the mistake of paying him at the point where I was satisfied with his work but the job was not just finished. I haven't seen him since. He left a small area of metal lath exposed without a scratch coat of mortar over it. (About one square foot) I can't get anybody else to come in for such a small project. This area is in the firebox (gas log). He left a bag of mortar and some sand. It's B type mortar. Does anybody know the correct mixture of mortar to sand to complete the scratch coat? Any help or tips appreciated. Thanks, Snappa--
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