screen voltage higher than plate

[loudthud]

If you can use an oscilloscope to look at the plate and screen without burning up your scope probes you will see nice quiet DC on the screen due to the choke filtering, but on the plate you see 10 or 20 volts peak to peak of ripple. That ripple goes above and below the screen voltage, your DVM just takes the average. So there might be 353V of nice steady DC on the screen, but the plate will have a sawtooth wave from +359V to +339V. The plate current remains fairly steady despite the varying voltage on the plate. The screen reading a slightly higher voltage than the plate should not concern you.