Open the map and measure in pixels the distance between beginning of a register (a line) and the next register beside it. Let's say you get 80.
Divide your measurement in to your target size (50).
Thus:
50 / 80 = .625
Scale the map by that amount. (.625 or 62.5%)
I don't like counting squares. I get the count wrong too often for taste. =) Glad it works for you though.