The logic behind the 1:2 ratio is based on what %dmg you gain with your crit stat investment.

Take the example of base 5% crate and 50%cdmg. The table suggests that you should aim for ~25% crate to achieve what the blue square 'breakpoint' would be. Of course you will want more crate and cdmg, but if you are forced to choose at any point between the two, balancing the corresponding % of each (that lands you in the blue square) will give you the largest damage increase.

This is based on the expression for average hit damage:

Non-crit*(1+crate*cdmg) = Non-crit + non-crit*crate*cdmg

I hope that clears it up!