Replacement window balancing coils for Certainteed vinyl replacement window

One of the balancing springs broke and I would like to replace the pair. There is no number stamped on the spring. I checked both sides after unscrewing the tongue end. It is 3/4"inch wide. The sash measures 27"x30", but I don't know whether or not it is double strength. There are two panes of glass with a clear thermal barrier in the middle. Weighing myself with and without the sash I found it to weigh 16.5 lbs. and I guess the margin of error is about 0.50 lbs. Would you advise 8 or 9 lbs springs?