50g per liter of water is what I use, and it has worked perfectly so far. It's quite a strong solution, and I can't imagine the need to use anything stronger.

'How much to use for heavy de-scaling?'
Citric acid is good for most de-scaling, but it will 'destroy' most zinc or chrome plating, remove most paint, etc. It is 'safe' for brass and copper. I do not bother at the concentration and simply pour a variable quantity in boiling water, maybe from 5 to 10 tbs per liter.
For a very dirty boiler, which has a lot of scale, sometimes I will use a mixture of muriatic acid and hot water. If you intend to use muriatic acid, do it outside your home, never inside. Some person are very sensible to the vapor and smell of it and can actually faint. Use glasses and gloves, muriatic acid is quite powerful for de-scaling.
For group heads, since most of them are plated, I use a coffee cleaner, such as coffee clean.
These recipe are intended for de-scaling copper pipes and copper boiler after a complete tear down of an espresso machine, they are not intended for occasional (maintenance) of most pour over home machine. If you try this in a home super-automatic machine such as Saeco or Jura, you will simply 'kill' them. You could still use citric acid but at a much milder (concentration) level one or two tea spoon per lit maximum. Never use muriatic acid in a pour over home machine.

FYI, the approximate weight/volume equivalence for citric acid with the consistency of granulated sugar is:
1 Tablespoon == 14g