you're just going to have to try it and see.

OR, find someone with a reloading scale. Measure 10 of your beads, then keep chopping at a piece of your lace until you get to the same mass as ten beads. Measure the length of the lace, then divide by 10, and you have the length of lace equaling the mass of one bead.

The reason you use ten (or more) is that trying to measure one bead and one piece of lace would be a big pain in the a$$ and also it puts the amount of mass somewhere out away from the lower limit of the scale, which is just easier to work with.