Roughly Ca 350-450, Alk 8-12 dkh, Mag 1250-1350 and SG 1.025-027. Ultra low nutrient systems are going to run a lower alkalinity. Those are range of values, everyone runs their systems a little differently, aim for consistency, avoid swings. Clams as they grow tend to really untilize the calcium.
There is tons of info on keeping clams that is stickied in the clam forum here at Reef2Reef, I suggest reading this one first and then some of the others.