Spiral Balances with a length of 24-1/2"

I need to replace some spiral balances. My balance is a blue tip (900 series) but the length is 24 1/2 and the diameter at the blue tip is 1/2 inch which is a spring and not a tube.(Note image) My balance is a combination of spring and tube. The spring at the bottom is approx. 13" and blue tip to end of tube is 24 1/2 as stated above. Which balance should I order....24" or 25"?

Just a note-----The bottom end looks just like the 900 series which is the 3/8 dia. and not the 600/670 series 5/8 dia.

Thanks for the reply. I just want to make sure you think the 24" balancer will work. I'm in between sizes. Look at the new photos and tell me if you think thew 24" is good. The blue tip won't move so these dimensions on my ruler are what they are. Thanks for working on this for me.
Rich M.

Thanks Paul.....Everything is clear, I will order the S600-25.
Thanks again for the help.
Rich M.

Paul, Thank you for your help with your recommendation on my spring balances. It was right on and your service is great. I ordered them (S-600-25)on 8-25-11 and got them on 8-31-11, installed them in two windows in about 10 minutes each and no more holding up windows with sticks. Great company and I'll recommend to all my friends. Again, thanks for all your help. Rich