Im a bit different, but i make my personal carts at .5g into 1mL and ive seen them as low as .1g in a cart, but i think its kind of a ripoff. .2 is pretty common as well.
At .25 one takes a sip just to kind of establish the strength, and take 1 or 2 good rips. Never had a breakthrough but i bet its possible with 4 solid tokes, but also, ive never seen anyone even try to take a 3rd toke.
I think its a good strength for most experienced psychedelic users.