The right-hand sketch relates to the carriage class. Dimension "b" will be:
ITA/ISO class 2 - 16" (407mm)
ITA/ISO class 3 - 20" (508mm)
ITA/ISO class 4 - 25" (635mm)
The left-hand sketch will help determine your fork's drop, compare "RH":
ITA/ISO class 2A - 21.65" (550mm)
ITA/ISO class 2B - 24.61" (625mm)
ITA/ISO class 3A - 25.79" (655mm)
ITA/ISO class 3B - 30.71" (780mm)
ITA/ISO class 4A - 33.27" (845mm)
ITA/ISO class 4B - 31.89" (970mm)