Spiral Balance Measuring Question

I am replacing damaged spiral balances for many of the windows in my house. I know which pivot shoes to order, but need some help with the spiral balance. I currently have the Red tip series 600 balances installed in my windows. When I measure the rod it comes out to 28 3/8. This is measuring from the top of metal rod to the bottom and it does not include the red tip or end piece of spiral balance. Should I go with 28" or 29" spiral balance? The other question would be should I stick with Red or is it okay to upgrade to the Blue tip spiral balance?