calcium in rain water

[gngrhill]

I have just recently gone to just rain water to water my plants. I used to alternate with tap water for the calcium. I bought cal-mag from Ray a bit ago, and need some help learning how to use it. Do you use it everytime you water or just once or twice a month ?
Also, I have read that calcium is very important for Cattleyas, and I would like to know if all orchids need it or is it just mostly Catts ?
What about Paphs which , from my understanding, like pure water. Would they also benefit from added calcium or would that be too much salts for them ?
I appreciate any input you can offer.
Thanks