I have one of them flasks for my flash pan. And yes, it is a great pan primer. Personally it pours a little too much powder for my tastes. So I just put in what I think is right and open the gate and let the powder fall back into the flask.
As for the flints, I believe I used 3/4x3/4 in my Lyman and as I recall they were a tad short. You may be better served with the 3/4 x 7/8"