sash balance replacement

I have non tilt double sash wood windows that are in good shape, albeit 30 yrs old. I need to replace the old balance springs but thought a spiral balance replacement would do the trick. Both upper and lower sash have an overall length of 38 inches and weigh about 21 lbs. The groove in the side of the sash is 5/8". In looking at your site, I thought the Series 500 with a foot bracket might work. Could you suggest an appropriate spiral balance length. I figure the lower sash balance would need to be maximum length, the upper could be shorter?

Thanks for assist,

Jack

Am I on the right track

1st window glass dimensions:L33 3/4"by W27 5/8,approx wt 28 lbs.

2nd window: L30 3/4 by W27 5/8, approx wt 25lbs.
These wts are based on a calculated method found on a website. Maybe you have a method based on dimensions and double pane glass.

The window has a tube spring currently.
Thanks
Jack

There are two different sized windows:

Window 1: height of glass 33 3/4, width 27 5/8 (both upper and lower sash.

Window 2: height of glass 30 3/4, width 27 5/8 (both upper and lower sash.

Please recommend based on these dimensions.

Thanks

Hopefully, last time. Each of window #1 and #2 have an upper and lower sash of the same dimension. Could you spec the the balances for each window for both lower and upper. I expect the upper sash balance would be shorter than that for the lower sash balance.

Thanks again