Grumpy_Mike:
You don't have to do anything to adjust the sketch for any size encoding disc, you are counting revolutions. You only need one hole in the disc, having more gives more pulses per revolution.
I'm not sure what you are having difficulties with, you might be over thinking things.
Seriously! Just one hole?
Maybe I am over thinking things, I've just seen more complex encoder disks when searching online.
How does the arduino know the location of the disk with just one slot?
I'll give it a try, thanks Mike.