Just to be on the safe side I took out the window and weighed it. The window weighs 30 lbs which, I suppose would explain why the 2910 option didn't work.
Yes, the spring strength is very important. For future reference, you should always remove and weigh the sash, just in case.
So I am guessing the 2950, 28-32 lbs. would be the proper spring strength option?? At least I hope so because that is what I ordered earlier today. If not please advise.