OOPS! I just saw my error. Looking at the bridge a little closer, I see that the bass side is a little further "south" than the treble side. So the torpedo level on the bridge idea won't work because the string should be a little longer on the treble side than the bass side to the tailpiece. You can still find the center with string length, but I think you'll need to put the torpedo level on the nut, and measure the string length from there. A bit more cumbersome, but it will still work.