Ideal values for the lower side of POT5:
300V - 0.44K
250V - 0.53K
200V - 0.66K
150V - 0.88K
100V - 1.32K
It may be close-enough to just pick a voltage goal and use an appropriate fixed resistor. The tube doesn't care if it comes out 230V or 260V.
I do not know if real parts will kick-up as high as 300V.
Unless you need under 133V, a 1K pot is ample and won't be as fussy to adjust.
Keep T1 away from strong heat. The output voltage will drop as T1 gets hot. With T1 at ambient, "250V" will wander from 254V on a hot summer stage to 245V at a winter concert. That hardly matters, but lean T1 against a tube or power resistor and it could drop from 250 to 210. Even that is no big deal except if you take voltage readings and switch off/on a lot it will drive you crazy.
> the trimmer controls the CV of the 555
Actually the trimmer directly controls the base voltage of Q1, which yanks the CV.
T1 will shift from full-off to full-on in a narrow range around 0.6V.
So if all is well, T1 base will be very near 0.6V, and you can predict divider (220K+pot) ratio from 0.6V against desired output.