If you have one of the vertical lines of longitude visible then measure along it. 1 minute of latitude = 1nm (don't have a half mil to hand to see what the graduations are.
Don't try measuring along the horizontal lines 'cos it don't work (unless you happen to be at the equator).
What I do though is carry a plotter with a half mil scale on it tucked into my kneeboard.
Isn't there a scale on the Frequency chart that comes with the half mil or am I imagining it?
Mike