Two possible answers:

1) It's doubtful that the difference between 2900 and 3600 (25%) is critical. Few things in the pool industry are that exact.

2) Call AquaRite tech support, and then try to break through to a level 2 tech (not the first person who answers). Ideally, you want someone who helped write the manual, rather than one of the guys who's flipping to the right section, and then reading the answer to you. Ask them about calibrating your unit's read out.

Ben