You should scale the reference drawing to ArchiCAD's model. Use the Resize command to do so.
1. Find a reference line or object in your DWG that you know the length of. In the example below I have chosen the inside of a wall that I know should be 15 feet long.
2. Next I draw a line in ArchiCAD that actually is 15 feet as ArchiCAD recognizes it.
3. Then I select all elements in the DWG and click the Resize button
4. Then click on the lower point of the inside of the wall
5. Then click the upper point of the inside of the wall
5. Finally I click the end of the 15 ft line I drew in ArchiCAD.
This will scale the DWG up to the proper size in ArchiCAD. Now my DWG elements are at the same scale as my ArchiCAD environment and no conversion is necessary. It is one to one.
You should also check out these recorded webinars about importing DWGs into ArchiCAD. There are steps you can take to ensure your DWG comes in at the proper scale the first time.
http://graphisoftus.com/events.php
HTH