lead screw pitch

I have been trying to figure out what encoders to get for my mill. The
mill has ball screws in it. I put a dial indicator on the table and
got .316 per revolution. This was not a sientific measurment it could
be a few one way or the other. How do I calculate for the number of
counts of the encoder for a resolution needed. Is this lead screw
metric? If so will that effect a machine using inches. I will try to
get a more accurite measurment of the screw to make sure. Thank you
Don B