The idea is to have added resistance in parallel to the pot when you use the neck pickup.

If you have 2 x 500k pots (for a 250k load) you'll want to use a 250k resistor. (A middle ground would be using a 500k volume pot and a 250k tone pot, and then perhaps a ~500k resistor in parallel.)

Basically, you'll want to wire one end of the resistor to the toggle switch where you wire the 200SN, and the other end of the resistor to ground. This will drop the load when you switch to the neck pickup (and also the middle pickup) but it won't be in the circuit when you're on the bridge pickup by itself.