Forest health bonus

this bonus is always there for cities with tons of forest tiles. but for cities with 1-3 forest tiles, sometimes i see the bonus and sometimes i don't. how does the game calculate whether you get the bonus or not? i know that working the forest tiles is not necessary for the bonus, what else is a factor?

also, sometimes the bonus is 1, sometimes 2. that seems to be related to how much forest you have, but i haven't figured out when it changes

Forest in fat + contributes 0.4 health (rounded down) so;

3 forest = 1 health
5 forest = 2 health
8 forest = 3 health
etc. . .

aha now i get it! thanks