The Ives Catalog has a chart referencing what Opening Width range each size coordinator could be used on and common applications of equal pairs of doors.

To determine the size COR you need;

• Start with the active door width (X)
• Next consider the overall frame opening between the stops (F)
• Preferably, the coordinator would equal the active door width (X) + approximately ½ inactive door width (Y).
• The coordinator must be at least 6" longer than the active door width (X) and less than the overall frame opening between stops (F)

In the case of an equal pair of 36" door (X = 36" and Y = 36");

• The preferred application would use the COR52 because that is closest to X + 1/2Y or 36" +18" = 54"
• At a minimum, the coordinator would need to be at least X + 6" or 36" + 6" = 42", therefore the COR42 could work, but is more likely to interfere with the latching hardware on the inactive leaf.