That's exactly the calculator I also wanted to link to.
Good that you mentioned the fact that calciumchloride has this habit to attract H2O from the air, which results in less calciumchloride per weight the more water has been withdrawn from the air.
For that reason, i would just try to shoot for 65ppm which happens to be 0.25g calciumchloride per liter water. We would er on the safe side and even if no H2O would be attached to the CACL2, we would still be in a very reasonable range from a brewing water perspective. For testing purposes, it might be interesting to shoot for even more, to make possible effects a bit more visible.
For my beers, I generally try to go for about at least 100ppm which translates to 0.33g/l. I think this might also be OK for mead, but I am guessing.......