The day goes by so much more smoothly when you think like the milling machine.  Perfect preps and good imaging leads to restorations dropping in at seat.  Here are some pointers for partial restorations with regards to fit:

1. Make sure your exit angles have a slight flare taper so that you can see all point angles in the boxes

2. Make sure the isthmus at the occlusal table is wider than the isthmus at cavosurface when you have a lingual or buccal finger / extension.  Otherwise, the bur has to mill back on itself and it can shatter the restoration

3.  At the path of draw view, make sure you see an uniterupted blue line like the image below.  If you don't. then that means there is an undercut that will be milled and you won't be able to seat the restoration.

4. The yellow shadow is a good warning also if you have undercuts.