300yds with a 6x42 is probably not an issue, but the problem is the bullet drop, and unless you have a quality BDC installed (which is unlikely on 6x42), on a fox, you'd need to take POA well in the air above it's back.
If I took a shot at a fox at 300yds with my .243 and 100g norma sp, I would need to aim 14.5" above my POI, and say with 10mph full value cross-wind, another 12.6" right/left. all in all, I'd need to aim somewhere randomly in front/behind and over the fox in the air.
So to summarise, my max range, even though I can shoot much further, is 225yds for a roe/red, as my aim is still on the top/back of the deers body, so there's no question about guess-work with holdovers. and for fox, max range would be 200yds, as 3.6" drop, meaning a back-line hold.
just my opinion.