Very cool! Been doing that for years. Back when you actually had to buy and read books (yes, I'm a dinosaur and that old

).
I've always used a similar method but use voltage to determine polarity of a coil or the pickup as a whole and a cheap compass to determine magnetic polarity.
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I use a compass perpendicular to the coils to determine the magnetic polarity of the coils. As opposites attract which ever end of the needle is pointing away from the coil is the polarity, if the north end of the needle is pointing away from the coil is is a north up coil or vice versa. The following illustrates a north up magnetic polarity.
Attachment:
single-coil-pickup-polarity-depicted-900.jpg [ 273.63 KiB | Viewed 84 times ]
Set meter to 20K ohm setting to determine which which wires are paired (start and finish of each coil). You will only get a reading with pairs (start and finish of a coil).
Set meter 20V to determine which wires are start and finish
Clip negative (black) meter clip to one of the pair, the red (positive) meter clip to the other.
Touch screwdriver shaft to the coil pole pieces then pull it off. If it goes negative first start wire is negative clip, positive first start wire is positive clip.
Slug pole pieces is usually north up and adjustable south up but there are exceptions.
E.G. north start (hot) --> north finish --> south finish --> south start (ground) or the opposite (south start is hot and north start is ground).
This is how most diagrams I've seen are depicted.
With existing pickup set meter to 20V and clip leads to hot and ground with volume and tone wide open. Again touch either coil and make note of whether it goes negative or positive first.
Test the new pickup the same way and if it goes positive or negative opposite the existing pickup leave the splice between north finish and south finish connected and reverse hot and gnd. Now the new pickup should go negative or positive first like the existing pickup and is in phase with the existing pickup.