In SilverFast there are two main scan modes. One called raw which export a DNG file and one TIFF. In the DNG format I need to invert as part of the post process in Lightroom. Or I can go for TIFF format and let SilverFast do the 'magic'. I would prefer scanning in DNG format but i am not sure how I can best invert a color negative. Any advise?
Your question has two parts and each part has already be answered (at least partially). With a DNG image out of the scanner, you will have to invert the color. Then, regardless of the image format (DNG or TIFF), you will maybe observe a color cast.
How to invert color in Lightroom
You can have a look at this post : Is there any good method to invert a negative image (duplicated with a digital camera) in Lightroom?
Using Lightroom, go the Tone Curve adjustement section. Make sure Linear is selected (Point Curve) and that you are using the Channel RGB. Now invert the curve by moving the two extremes points.
How to adjust the color
Have a look at this question : I have scanned a film negative - how do I adjust the color using software?
If your inverted image has a color cast, the easiest way to correct it is by setting a neutral (gray) spot in the White Balance Selector.