1. It is usual to adjust the length of the spout on the flask (cut or file it) to deliver the desired powder charge. Of course you first have to know what the desired charge is. Determine this by weighing charges into vials, and experimenting until you find a charge which works for you. Then you can adjust the spout to deliver it. Actually I don't bother with the flask at all. MLAIC, MLAGB both forbid charging of guns directly from a flask as there is a risk of an ember in the chamber setting it off. (I know a chap who lost the tip of his trigger finger when his tiny priming flask flask blew up while charging the flash pan of a matchlock). So if you are anyway going to use vials, then why not simply weigh up all your charges at home and forget the flask? I use a Lee Perfect powder measure to throw the charge. This said, I see plenty of folk who do charge directly from a flask, and I also see plenty who fill vials from a flask on the range. Whatever works for you.
2. It's a matter of experimentation to find the amount of filler. What you are trying to achieve is to get the ball just below the mouth of the chamber when the charge is light-to-medium compressed. Having worked out how much by trial and error for one particular BP charge, you can quickly work out how much to adjust the filler for different BP charges. I weigh out filler into vials as well - it makes loading on the range faster and simpler, but it is acceptable (in fact more common as well) to dispense filler from a flask directly into the chamber. Suggest that you start with too little filler and work up to it. Too much means that the ball will be proud of the chamber mouth and then you can't rotate the cylinder - and it is a struggle to remove it.
By weighing out both BP and filler into (separate) vials, it is easy to take a variety of loads with you to test in a single session.
3. "121036930043" These look a bit expensive to me, but that is the king of thing. Kranks do a kit. There are other styles of vial on the bay.
4. Don't like Pyrodex - I had problematic ignition with light charges (hang fires). Never tried 777. Once you try BP you will not want to use substitutes.
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TANSTAAFL