Hi Tom, also you can calibrate MoI in a similar way like Colin describes above.
Something like - draw a 10 inch line, then zoom until your line matches 10 inches on a ruler and now you're calibrated and any dimensions that you draw will be at real size until you alter the zoom or viewport size.
This is similar in concept, but adds some bells and whistles.
It's Free
IC Measure provides powerful tools for the manual measurement of circles, lines, polygons and angles. The convenient user interface enables the easy measurement of lengths, angles and surfaces directly from the image displayed on the monitor. Measurement data can be exported as CSV file.