To set your INSTEON dimmer module's On-Level:
Use either the local buttons on your dimmer module or a Linked INSTEON Controller to adjust the light to the desired brightness.
Press & hold the On button on the dimmer module or Controller to brighten the light. Release at the desired brightness.
Press & hold the Off button on the dimmer module or Controller to dim the light. Release at the desired brightness.
Once the desired brightness has been achieved, tap the Set button on your dimmer module once.
The load will flash. Some INSTEON models will beep once. The local On-Level has now been set.
Test the On-Level settings by pressing the On/Off buttons on your dimmer module.
For full reference, use the manual:
http://wiki.smarthome.com/index.php?title=2476D_Manual_(Rev_5.0)#Setting_the_On-Level
Note that if you link within 4 minutes of setting the level, it will be used as the link on-level instead, so don't link it if you're trying to set the local on-level (as you are).