[Alex KL-1]

Good, so is possible to add the resistor.
BTW: I wrote "in the wiper" (perhaps making some confusion), so I can explain better: the resistor needs to be added in series with the ground side of it; the pot. pin dedicated to low voltage, and potentiometer being only a part of the divider. Like a 3M3 resistor in the ground side, then to the one leg of pot. The movable part of it goes to focus grid, and remaining part of the potentiometer simply goes to 530V. Total, more than 4M, better than the only 1M from potmeter.