I do 20 percent about every 1-2 weeks. I am using scripps iso water from the ocean and mixing with rodi. alk is 7dkh and cal is 340, ph 7.8. my parameters stay the same with nitrates,nitrites, and phos, at 0. I know that it should be taken care of by water changes but its not. I got some phbuffer and reef complete both seachem products.
i would say your Ca and alk are about right. there is little reason to run higher numbers than natural sea levels. alk at 7, Ca at 400. i would not worry much about changing any of those levels with the corals you have.
trying to keep a mixed reef is a lot more difficult than trying to keep either a nutrient poor system (for SPS) or a nutrient rich system (Xenia, Kenya tree, Zoas, and Ricordia). the nutrient levels that the softies will like, will be toxic to the SPS. it is very difficult to maintain that perfect knife edge where the levels are acceptable for all, but optimal for none (except maybe the LPS).