In my experience, a full staffed forester can just about keep up with a Lumber yard for fire wood when planting in fertile soil areas. Standard or barren soil they seem to have no chance.
My personal "ratio" is 2:1
I run 2 Foresters for every 1 Wood resource building (firewood and planks) the Sawmill absolutely destroys forests when fully staffed so having 4 guys planting trees in 2 separate foresters can just keep up with it.
Never have firewood and planks collected from overlapping work areas, it will decimate the forest area and will both run out very quickly.
Handy side note, you can use the forester to boost your trapper efficiency up to 100% if your struggle to do so, get it to plant trees in your trappers work area and you'll...