Balance strength option

Recently ordered a pair of S385-21, option 2010. Turns out to be too weak to hold the window up. There was a code on the old balance which appeared to say "2010" so I thought that was the correct option to order. Foolishly I never weighed the sash until now, and it weights 20 lbs, suggesting the "2040" option. My question: is ordering by sash weight the foolproof way to determine the option?
Thanks,
Stuart

Thanks, Paul