Best Practices for Indexing (ebook) by ASI (American Society of Indexers)
Continuing numbers — also called inclusive numbers — are numbers in an Index that show 2 or more pages, for example: 23–38. Continuing numbers are used when the entry on one page flows over to the entries of following pages — but not used when the entry on one page is not a part of the entry on the next page. In that case, we use the separate numbers (23, 24, 25) instead of the en dash (23–25).
When the numbers are truly continuing/inclusive, there is an indexing formatting quirk that asks us, in many cases, not to use all the digits in the second number.
Here is the recommendation according to CMOS 17, showing the four possible case
A. First number … is Less than 100
Second number … Use all the digits
Examples: 3–10 / 71–72 / 96–117
B. First number is 100 or multiples of 100 (200, 300, etc.)
Second number .. Use all the digits
Examples: 100–104. / 1100–1113
C. First number is 101 through 109, or 201 through 209, etc.
Second number … Use the changed part only
Examples: 101-8 / 808-33 / 1103-4
D. First number is 110 through 199 (or 210 through 299, etc.)
Second number: Use two digits — unless more are needed to include all the changed parts
Examples: 321–28 / 498–532 / 1087–89 / 1496—500 / 11564—615