There is no need to weigh the pan charge - no matter how much nor how little powder you put in the pan, will not effect the velocity of the shot, however, whether the pan ignites the main charge or not, might depend on how little you put in the pan.
I always fill the pan.
Others just cover the bottom of the pan, a few kernels deep - I fill it - when the pan goes off, so does the main charge. I VERY rarely get a flash in the pan - if my pan goes off, so does the main charge.
Once loaded and the ball seated, powder granules are visible at the vent.
You can use 3F - many do, or 4F as many others use. Your choice. You may also use 2F- some do. Note, the finer the granulation, though, the quicker the ignition, as shown in actual timed tests.