How could one be certain if a Swingweight scale is correct without a calibration stick.

I understand we could compare results with another scale, but if they were different, how would we know which one is wrong…. both could be wrong.

Is using the math formulas, several of which are online reasonably accurate, assuming ones measuring skills are precise?

P.s. i dont have a scale.

Thanks! I guess on one hand it doesnt matter, as long as all the measuring is just done on one machine, including the target number one wants to get to.