Bill Lawrence always said two nickels below the strings fretted on last fret. Then adjust the bridge pickup for even output using your ears. It's a pretty good rule of thumb to avoid magnetic damping of your notes.
Since those soapbars don't have angle adjustment, I would measure the nickels in the middle of the pickup: that's where the mags live and where the mag field is strongest.
Since those soapbars don't have angle adjustment, I would measure the nickels in the middle of the pickup: that's where the mags live and where the mag field is strongest.