Use all digits for the second page number if the first page number is any number from 1 through 99. For example, 7-15; 37-39; 3-113. Use only an en dash between the first and second page numbers.
Use all digits for the second page number if the first page number is 100 or a multiple of 100. For example, 100-107; 200-300; 1300-1311. Use only an en dash between the first and second page numbers.
Use only the changed part of the second number if the first number is any number from 101 through 109, 201 through 209, 301 through 309, etc. For example, 301-7; 703-9; 1206-8. Use only an en dash between the first and second page numbers.
Use two digits -- or more if required -- for the second page number if the first page number is any number between 110 through 199, 210 through 299, 310 through 399, etc. For example, 113-17; 227-30; 1077-78; 11733-800; 32977-33000. Use only an en dash between the first and second page numbers.
Use all four digits for the second page number if three digits change in a four-digit number. For example, 1500-1611; 1723-1833. Use only an en dash between the first and second page numbers.