its not an exact science. Just toss it in and then pull it out in a few days.
There really isnt an actual number set in stone for letting it reach before removal.
.25 will work, .5 will work. Doesnt really matter. All you want is so introduce something that is biologically breaking down and starting the life cycle for benificial bacteria.
I think the above posters already nailed it but just wanted to toss in the info about the waiting game.