Is there a reliable way to take the length and width of an object to be covered, and the diameter of the cordage being applied, with a flat Portuguese sinnet and determine the length of cordage required? I guess the math would be pretty simple if you knew the variables but I don’t know the formula.
I suppose it would also depend on how tightly one applied each turn of the knot, but there must be a way to guesstimate the average.
Calculators for similar things have been developed.
But factors such as your personal preference to tightening, and the elasticity of the material used, will speak for the method of tying a defined length, then measure the used goods. Add a bit to pull with when you come to the end. And don’t be shy with it.
Sometimes, such as with the French sinnet, the start differs from the run slightly, and it could be wise to make different tests.