There’s a point below which there is so little sealant in the tyre that it’s spread out too thin to seal a puncture, so don’t be too stingy. Two Stans scoops per tyre for me too.
As to the when part of your question, as late as possible in the tyre fitting process, so ideally, just before fitting the last part of the second tyre bead.
Two stans scoops in a mtb tyre for me. Probably a bit more than needed but I’m not a racer
+1 the little bottles don’t seem to have the gritty stuff the same like the bigger Black bottle. Don’t think of the bigger outlay of buying the bigger one because it’s a false economy. By the time you’ve bought 4 of them it’s the same price of the bigger bottle and you’ll get twice the amount. And the better product
I use one scoop a year on from starting with tubeless and it seems OK.
Use a tube in the tyre first to pop the beads across, take it out one side leaving one bead still seatedm
Put valve in and seat second bead.
Remove valve core (if its a removable core) and using a syringe inject fluid
Take as little of bead of one side of rim and pour it in.