Hi Benny,
Your understanding should be correct. Do you have a multimeter? If yes then you can easily check
1. Switch in far left position - measure between far left and center terminals. Either you get a zero reading (then this is the "on"-position of the switch) or an infinite reading (then it would be the "off" position - in that case you would get a zero reading between the center and far right terminal).
2. Switch in far right position - if you do the same measurements you should get the reverse results from Step 1.
3. Switch in center position - you now should get a zero reading between left and center terminals as well as between right and center terminals (both pickups "on").
If you don't have a multimeter, get one. They are not expensive and IMHO indispensible for serious guitar wiring work.
Cheers Stephan