8.4v means no boosting.411 is too low. .400 to .475 signify bad feedback line to lp8550. 0.511-0.545 is what you want after the diode. What does ball A5 for feedback on lp8550 look like?
Also, what is your backlight voltage? 0v, 8.4v, 19v, 27v, or 49v?
0v means break in line or being told to stay off altogether.
8.4v means no boosting, but continuity all the way through on the top of the line from coil to diode, diode to output, etc.
19v means missing pwm or bad lp8550
27v means we're blind
49v means bad lcd/lcd backlight cable inside assembly is not plugged in/bad cable.