boosting CRT heater voltage

Hi all, I suspect some folk here have done this before.... :-) My desktop CRT (over 10 years, but not what I'd consider vintage!) has been ailing for some time, with the picture getting darker and darker despite having the brightness at 100%. It's still good for high-contrast stuff such as black text on white background, but forget trying to pull details out of most photos, for instance. Heater voltage seems good at 6.4VDC / 350mA (it's derived from the PSU in this monitor rather than the flyback section), but I'm considering boosting it a little and see if it improves things, obviously shortening the life of the tube in the process. Question is, what's a sensible amount to over-run things by? Say I aimed for around 10%, is that too much and going to kill the heaters in next to no time, or so little that unlikely to really make any useful difference? cheers Jules