Chapter 1:  3 sentences
Chapter 2:  5 sentences
Chapter 3:  4 sentences
Chapter 4:  6 sentences

The chapter with the longest chapter is 4, so we pad all of them to 6

Chapter 1:  01, 02, 03             # 04, 05, 06 are padded
     real:  01, 02, 03
Chapter 2:  07, 08, 09, 10, 11     # 12 is padded
     real:  04, 05, 06, 07, 08
Chapter 3:  13, 14, 15, 16         # 17 and 18 are padded
     real:  09, 10, 11, 12
Chapter 4:  19, 20, 21, 22, 23, 24 # no padding
     real:  13, 14, 15, 16, 17, 18

Now let's say someone says, what chapter does 15 belong to?

ceil(15 / 6) = 3

Now let's say someone wants to know the true ordinal overall value

my %pad = (1 => 0, 2 => 3, 3 => 4, 4 => 6);

15 - $pad{ceil(15 / 6)} = 11

Now let's say someone wants to know what is the ordinal value of the sentence within the chapter

15 mod 6 = 3