McMorran balance - how accurate?

I am thinking about getting a McMorran balance or one of those fishing line things that measure your yarn - the McMorran seems to be much cheaper, but I am wondering how accurate it is? Anyone have an opinion or firsthand knowledge?

I am spinning some white alpaca in laceweight to make a lace blanket for my grandson due to arrive in December. Haven’t chose a pattern yet, but I want to know how many yards I have so I don’t run short.

TIA,

Deb in N TX