The least amount of powder theoretically that needs to be poured into the powder tube is just enough to fill the powder bushing, but why would you want to do that? I would recommend you put in at least enough powder to be visible in the tube. You'll find the P-W tubes are easy to drain and bushings can be changed with the tubes filled with powder and shot.
It's good practice to weigh out your powder charges with each lot of powder, you will find that they will vary a little. I have found that my P-W powder bushings in general, drop a couple of tenths less than the chart says they'll drop. I wouldn't be overly concerned with how long the powder sits in the tube--unless you're talking months or even years.
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Wild Skies
Since 1951
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