I just found out about an excellent article on borate chemistry in the September 2010 issue of Pool and Spa News here. The article talks about borate sequestering ability, but this is very weak so can pretty much be neglected as far as the saturation index is concerned where 50 ppm Borates would lower the index by about 0.02 units assuming you allow for the 5.2 ppm TA increase (i.e. keep the carbonate alkalinity the same).