*Note: this is from my experience and research on the web.
HHmmm, this is a pretty tough question to really hit with the right answer.
I use a 3:1 ration, but I use RO/DI water. I also add seachem so that might help too.
But If I was going to raise 50 gallons 5 GH, I would add 11.54 CaCL2 and 13.46 Mg.
Now, this is the funny part
A) There are different types of CaCL2, so depending on what you add the results may vary.
B) You could actually add too much CaCL2 or Mg, so I would get a calcium test kit and test them. When you know what your Ca & GH is, you can back in to what your Mg levels. (I'm a geek, so I like testing to give me a better feel for the tank) Just because your PH is 5 when it comes out, this could mean you have a crazy amount of GH, but no KH.
. Too make matters worse, if you have too much CaCL2 or Mg, this also cause Iron uptake issues.
. I figured since I add K, NO3, and P all separately, I also have a solution of Ca and another bottle of Mg. But I really only test my GH once every three weeks when I do water changes. Quick note: I tore out all my quick growing plants and now my tank consist of slower growing plants. This allows me to do water changes once every 3 weeks (I've only had this set up for 2-3 mths, so it's still in the 'test' phase)
C) The good news is this is a fish tank, not rocket science. Don't worry about getting exact measurement. The tank will accept a nice margin of error too.
Hope this helps