IF you can find a light which runs on battery power, it will not be effected by the fluctuating AC line and should provide a more accurate reading. Also if your strobe disk is small enough, you should try to play a dynamic LP as you are checking for speed by placing the stylus in the outer grooves. This will allow you to account for stylus drag.
I use the KAB strobe which works as described above, but it is expensive and some report not quite as accurate as the Timeline.
I use the KAB strobe which works as described above, but it is expensive and some report not quite as accurate as the Timeline.