The way I handled it was to have the bottom row of hexes from the Tors of Levenies to the East Sellen form the northern border of Mivon, while to the west started the borders of Pitax. I also had the top row almost all the way across the map belong to Brevoy.
You can check it out here.
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In the game I ran, I left it kind of vague. Mivon was okay with an undefined border as long as there wasn't a country up there, but once the PC's kingdom started growing, Mivon got more territorial. By about book 3, the PC's had to negotiate an official border with Mivon. Mivon started off claiming all the way to the Little Sellen River, but the final deal (after a series of modified Diplomacy rolls) ended up in about the sam eplace as Jedric's (i.e., at the bottom of the last line of complete hexes).
One of my players found this map, which I thought was really cool. According to it, Mivon claims the hills from the river to the forest, just below the Greenbelt map. But the forest itself and the other side of the river are clear for a bit before you hit Mivon's border.
For my campaign, I put Jovvox at the bottom of the map, between the stolen lands and Mivon - which is (perhaps) a few hexes away from it proper location. It was fun having a Gnome settlement in the game. My players hated the place and really did not trust it (or its residents) at all.
In our interpetation of the map, Mivon claims the bottom 2 or 3 hexrows of the map.
Pretty much everything below Candlemere lake.
Just compare the rivers in the River Kingdoms with the more detailed ones in Kingmaker.