Here is a paragraph from the old IPC-D-275 that provides a measurement.  
Could not find a similar para in IPC-2221

5.3.1.4 Edge Spacing Except for edge-board contacts,
the minimum distance between conductive surfaces and the
edge of the finished board shall not be less than the minimum
spacing specified in Table 3–1 plus 0.4 mm [0.016
in]. Printed boards that slide into guides shall have a minimum
external conductor to guide distance of 1.25 mm

Susan Hott
Robisan Lab

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