It totally depends on the size of your scoop and the type of food. I.e nuts is heavier than chaff. Put a scoop of feed in a platic bag and bung on some scales at home to find out.
Your best bet is to weigh out your 350g,then see how much of the scoop it takes up(ie 1/4-1/2 scoop)I Stubbs round scoop will carry anything from 0.25kg straw chaff to approx 1.8 kg of cubes,so you see,there is a stark difference based upon the density and type of feedstuff being measured.
I do the same as you -I feed a forage diet so don't have to be too precise, but I do initially weigh them by the scoop so I have a reasonable idea of how much I am feeding.