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I think a 25 Watt rating is gross overkill. I have never needed more than a Watt or two for the resistor.
I don't recall the forward drop of a selenium rectifier but if you know it, then you know the extra voltage you will have to drop in the resistor. Ohm's law is your friend.
However, since the current isn't steady or even sinusoidal, you will have some error, often inconsequential. If that bothers you, adjust the resistance to get the desired B+.
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