You can't; there only so many taps on the power transformer.
Wired for 235-245 just means it's wired for 240 volts nominal. Ten percent high is a traditional maximum deviation for tube gear, and perfectly safe in Stereomour for 90% of tubes. That would be at 264 volts. (5% would be 252v. 245v is only 2%.)
If your average mains voltages differs from 240v by more than you are comfortable with, then it's time to adjust the filament dropping resistors. That's what they are there for!