Why one rule cannot cover it
A teaspoon of whole chamomile heads and a teaspoon of ground cinnamon are not comparable quantities of anything — the densities differ by an order of magnitude. Volume measures are why the same instruction produces a weak cup from one material and an undrinkable one from another.
The three bands
Leaves and flowers: roughly one to two grams per hundred millilitres. Roasted grains: two to four, which is about twice as much and the reason grain drinks so often come out thin. Accent materials — lavender, wormwood, ajwain, lemon myrtle, kuding, elecampane — a pinch, and a teaspoon is a mistake rather than a strong cup.
Under-dosing looks like weak material
If a roasted barley or corn drink tastes of almost nothing, the material is usually fine and the dose is usually half what it should be. The colour of the liquor is a poor guide here: pale mugicha can be substantial and a deep red snow chrysanthemum is light.
Over-dosing looks like a bad plant
If lavender tastes of soap or kuding is undrinkable, the quantity is the first thing to check and the supplier is the last. A great many people conclude they dislike a botanical after meeting it at four times the sensible dose.