Hey, if you can, set the chiller in the res a little cooller than you want in the buckets because the buckets will creat a warmer water temp since they are under the lights and the fresh recerculating water from the res will off set that warmth. I read a thread yesterday that said 59D is the most proficient water temp for O2 max. Only thing, not all strains will take to temps that cool while others will thrive like no others. I would set the chiller at about 62D and test the buckets directly under the lights and if they are staing under 68D then you are in good shape. I would do the test after leaving lights on for 12 to 18 hours or even a 2 day trial, a test with lights on and lights out.
Hy gang, been busy so haven't had much time to post.