In addition to that resistor, add a .001uf to .0033uf cap that engages when in split or parallel. That further takes the edge off.
Say you have this on your push/pull:
1 2
3 4
5 6
Where 4 is the negative from coil 1 and has a jumper to 1, 3 is positive from coil 2, 5 is positive from coil 1 and also goes to your pickup selector or volume pot depending on the guitar, and 6 is ground. Ground of coil 2 can go to 6 or the pot casing, doesn't matter either is ground.
In the "down" position, 3 connects to 1 (which is jumpered to 4) so your pickup is in series (positive of coil 2 connected to negative of coil 1). In the "Up" position, the negative of coil 1 connects to 6 (ground) so it operates as it's own coil, 3 connects to 5 so hot of coil 2 connects with hot of coil 1, i.e. 2 single coils in parallel.
Now connect a 470k to 500k resistor and a .001uf to .0033uf cap from 4 to 3. In the "down" position, 4 is jumpered to 1 and so shorted to 3, the resistor and cap have no effect when the pickup is in series humbucker mode. But when you go to the "up" position, 3&5 (hot from both coils) short and now see that resistor and cap to ground (remember 4 shorts to ground (6) when switch is "up") in parallel with your volume pot. So if you're using a 500-550k pot, your two single coils in parallel now see an appropriate volume pot load for a single coil (250k ish) and excess high end is also shunted by the small cap.
Works wonders.