I’ve actually started using 20 foot lengths and just adding as needed. That’s the reason newer tutorials don’t have a fixed measurement for the rope.
I find it a lot easier to work with– at first I used huge bundles, upwards of 50 feet, and it was really unwieldy during the first part of tying.
Also, the flexibility of multiple lengths allows the tie to be used on any body; the amount of rope will change depending on the dimensions of the tied person.
The way to extend is to make a lark’s head (below) with the next bundle of rope, and secure that loop over the tails (end knots) of the rope that needs to continue.
I hope that helps!