Q: How much line is needed in total (for the two sandals)?
A: It's partial by the size of the foot, but when approximately 6 m can be prepared, I think it's enough.
По моим прикидкам 6 м веревки диаметром 6 мм это на 1 сандалию, следовательно на пару - 12 метров.
Q: Tried making one recently with jute twine of around 1.5 mm circumference. Took more than 3 hours to make. Clearly too thin. Have access to 3mm and 6mm alternatives. Can't tell which to use. Limited budget so can't experiment with both.
A: I will challenge you, very impressed.
Japanese technology at the time, to make a thread of 1.5mm, please consider it very difficult.
I do not think the comfort, I think the best would rope over 6mm.
If this is possible within 1 hour production time.