Deck lines

Looking for some new deck lines and found this. How much should I get for my 18 foot kayak?

Don’t forget to add a few beads for clearance to get your fingers underneath.

For my 17’ kayak I used about 70’ to be on the safe side. But I made double loops around the bow and stern. It’s a wood kit and I have no hole or attachment point at the ends to hold the deck line in place. This added maybe 4’. Knots take up rope too. But 70 feet of that would sink my kayak or float my boat depending on density.