The starting chain is a multiple of 4 + 1.
You might be wondering, what does that mean? How does it help you adjust the size of the necklace?
The basic stitch repeat is worked over 4 stitches. The + 1 is the additional chain needed at the end of the starting chain that substitutes for the first single crochet stitch.
In a nutshell, if you want to make the necklace band longer, you can add on to the starting chain in increments of 4 stitches. If you want to make the necklace band shorter, you can subtract from the starting chain in increments of 4 stitches.
How do you know exactly how many chains to add or subtract to get the measurement you want?
The key is using your gauge and a little bit of simple math to calculate the amount of stitches needed for the customized measurement.
The pattern, as originally written, calls for a foundation chain of 77 stitches. 77 –1 = 76. 76 can be divided by 4; (76 / 4 = 19. 19 is the number of times you will repeat the stitch while working the entire necklace band. If you work at the same gauge I did, 3 stitch repeats = 2". This will give you a necklace band that is slightly longer than 12.5" (12.67" is the exact size, to be mathematically precise.)
So, with that in mind, you can use this information as your starting point.
Let's say that you want to make a necklace that measures 14". In that case, you would create a foundation chain of 85 stitches.
85 – 1 = 84. 84 can be divided by 4; (84 / 4 = 21.) You'll repeat the stitch 21 times while working the entire necklace band. If you work at the same gauge I did, 3 stitch repeats = 2". This will give you a necklace band measuring 14."