Milgard window sash balances

I need to replace some Milgard window sash balances. Your 15" Series 380 Channel Balance (Product Code: S380-15) seems to exactly what I want however, their are no markings on the old channel pieces. I don't know the weight of the window. I could take the window out and weigh it but it is in a rental I own in another city and will delay the replacing at least a week. I'm guessing the window sash weighs about 10 lbs. Should I go with the 1430 (10-12 lbs) or is it critical that I get the exact weight of the window first.