SR with teleconverter

I have just acquired a Tamron 2X converter to use with the Sigma 70 -300 lens. Do I set shake reduction on the K10D for the actual zoomed lens focal length in the normal way for a manual lens, eg 260mm or set it for the focal length of the lens + converter, in the above case 520mm? Logic says 520mm, but..........
You may say 'use a tripod', but that isn't always possible!

Set it for the actual total focal length. (ie 2x the focal length of the lens if using a 2x teleconverter)

The SR mechanism needs to know the actual focal length in order to effectively compensate for camera movement.
Matt

Many thanks Matt