Since you've already read the primer on how
to make a coil tap, now you can create a variable
coil tap that you can dial in or out.
Here's the arrangement:
Solder the two "middle wires" (i.e., the two
normally soldered together in series) to the middle lug of the potentiometer.
Solder one of the other two lugs to the ground.
Which one depends on whether you want the coil to be dialed in or out.
Looking onto the three lugs with the shaft pointing up...
Solder the ground wire to the lug on the left
to have a humbucker when it's turned up (tapped when turned down).
Solder the ground wire to the lug on the right
to have a single coil when it's turned up (both coils when turned down).