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RE: I need to calculate the value of a voltage dropping resistor

The better way to do this is to use a 250V (2x 125V in series) zener from the screen grid to ground with a resistor from B+ to the grid. That gets you a basic shunt regulator.

At your operating point, the screen grid draws from 0 to 7mA, so size the resistor to run enough current through the zeners such that they keep regulating even at maximum draw - maybe 10mA - which means a ~5K (1W min) resistor.

To make it even better (lower noise and impedance), add a ~100u/300V cap to bypass the zeners.

A couple of glow tubes in series will work instead of the zeners, but they are noisier and have higher impedance and require more standing current.

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