My spiral balance measures 28-1/2" long

Hello! I'm afraid we don't have any spiral balances whose length measure on the 1/2". Usually that's not a problem, though. Based on your sash size, I would normally recommend that you round down to 28", but the window manufacturer used 28-1/2". There must be a fuller frame that requires a slightly longer balance. Can you confirm? I'm thinking you should consider our S900-29 balance with a red tip (for sashes weighing 10 through 22 pounds). Would that fit in your window or would that be too long?

For a 28" length, the spiral might not come down to the sill, so the window won't close by 1/2". For a 29" length, you might not be able to raise the sash all the way up, short of a 1/2". That's why I recommend the 29" length, since between not being able to close all the way and not being able to open all the way, I'd rather have the latter.