I have a lower quality scan, but in higher size, and in my copy the number for the scale seems clearly to be "1/10", but in your copy it seems to be "1/16".
However, that would also make sense because the periscope reticle has marks divided in mills and not in degrees, and the division is, according to the periscope manual I have, 1/16 of a degree
In any case, and no matter how you name the scale (If referring to the number of divisions, 1/10, or to the size of the divisions, 1/16) the end result is the same, and the formula mentioned before works the same. It's just a matter of terminology.
Cheers