yes add it when you get 3+ days of the same reading. I like to take the sample then that it 3 days later, that way you are not wasting beer every day. If its changed take another 3 days later as if you took one every day that would be 6 samples your wasted instead of 2 lol. onto finnings its best if you can cold condition (CC) the beers at a temp as close to 0 as you can get, add the finning when FG reached then CC for 2+ days the cold and the finning will drop the yeast out. Also do not rush finning and bottling, the yeast will only benefit from being left longer they will clean up after them selfs and once they clean up they will start to drop out of suspention this is when fining and CC will help them drop faster making the beer clearer