I am a bit confused. What makes you think the edge cut is not “precise” enough?
If you assume the zone does not fill because of an error in the edge cut then i have to tell you that this is not the reason. A zone in kicad only fills if it has a connection to a pad of the correct net. So you need to place at least one dummy component connected to said net for your zone to do anything. See How to create a power plane (using zones)