Thanks for posting this.
Note that SeaChem Reef (and regular) also have high borate that can through off the alkalinity readings. But overall it's one of the better ones IMO. I would use a SeaChem Alkalinity test kit vs. some others if you are using close to 100% Seachem salt.
http://www.advancedaquarist.com/issues/dec2002/chem.htm
Here is an exert from Randy's article:
However, if the boron level is substantially above natural levels, as it is in the Seachem salt mix with 12x normal levels, borate can actually begin to dominate such tests, 19 and makes knowing the real bicarbonate and carbonate alkalinity much more difficult. Seachem sells a special borate alkalinity test kit to try to disentangle these effects, but that is only really necessary with tank water that contains greatly elevated boron levels.