ok.
the resistors are within 3% of spec .
may be an issue in the feedback circuitry.
check the following diodes
D105, 107 - 113, 117
also check R108, C126.
although not a common failure, check C105.
replace C713 and C730
SHOULD have gone into shutdown before ever reaching 300v !
while in it , suggest replacing the following capacitors as well
C107,115,126,310,502,610,613,615,706,709,716,805 and 806