I'm not sure there's an ideal level, certainly not a ratio.
I've had tanks do well with virtually none, other I could lard it on with reckless abandon.
Limiting ppm's for one tank might be something very different for another.
I dose about 5 ppm per dose 2-3 x a week to a couple of tanks, one maybe .5 ppm.
The lower light, low demand, lower energy tank is fine either way.
Sediment can supply ample amounts if you use that as a source also.
Same with fish food/waste.
Depends a lot on the tank.
For a stemy higher light tank, 4-5 ppm seems to help and prevent GSA, particularly IF YOU HAVE GOOD CO2.