Sash Channel Window Balance Stamp #

Hi- I have a 23 year old home with custom single hung windows with sash window channel balances. The "stamp" on the originals look like this, depending on the size: #2740 28 or #2790 31. I took the sash off of the one stamped #2790 31 and the sash weighs 23 lbs. The pulley systems are old and the rubber cracked, making them difficult to open and close. I'm concerned about safety. How do I choose stamp option on my replacements? Weigh sashes and pick on that corresponds? What if in this case it's 23 lbs and splits two stamp ranges. if I go with the higher range 23-28 lb, will it still work or will it be impossible to close? I guess given the two, I'd be happier with easier to open and harder to close, than easier to close and harder to open.

Thanks.