Knot a Tube

I have been wanting to make a Celtic Knot cushion for some time. Found I could stuff as you go SAYGO?) the tube on my sock machine. Made too much, so shortened it and made the simple love knot cushion as well. The latter can double as a draught excluder! I would love to be able to do a more complicate Celtic knot, and there are certainly plenty of ideas on Pinterest, but it is so difficult to calculate the length needed.