The idea is to get the volume to be 50% air/ 50% water when the pot is fully saturated. you get that with 1:1 mixture with sphagnum peat moss. There's research to back this up and how much perlite or similar essentially equates to the above mixture of water/gas per volume.
Coco coir holds 2/3rds the amount of water per volume compared to sphagnum peat moss. Based on same 50%water/50%air target, that's a 2:1 ratio for coco coir. 70/30 is close enough and that's what it comes premixed at, so convenience over being anal isn't going to make a huge difference.
More will only increase frequency of irrigation with little to no benefit. i.e. just more work. Follow these ratios and it's damn near impossible eto overwater a plant... if you are it has to do with frequency. Let it dry more between in that case. remember, the plant does 95% of the work.. we just have to not screw up. don't try to force things. let them happen.