I don't know how far away from the border the fighters take off from. I also don't know how altitude plays into the whole combat and range thing. For all I know, they take off 300 miles away from the border, cruise at 30k feet, fly lower, drop some bombs, fly back up at 30k and head another 300 miles back.
I don't actually know if 600 miles is a long distance for an F-16 but it is a small plane.
I also don't know how big Ukraine is thinking about it.
The range is both very important and not applicable to your question. Ukraine fighters will always reach the war zone, whatever range they have, because fighters with less capacity will take-off closer to it and thus be more at risk.