According to instrument formula given by G.V. Black, primary cutting edge angle should be:
More than 50 degree
More than 90 degree
Less than 50 degree
Less than 90 degree
The second number of a four-number code indicates the primary cutting edge angle, measured from a line parallel to the long axis of the instrument handle in clockwise centigrade.
The centigrade angle is expressed as a percent of 360 degrees (e.g., 85 = 85% × 360 degrees = 306 degrees). The instrument is positioned so that this number always exceeds 50.

Sturdevants operative dentistry 7th edition page e2