That looks pretty good.
Only thing I would be concerned about would be, whats the field of view being used? Although thats not too huge a concern, So I dont blame him for leaving it out.
Aside from that, there are only a few possibilities for error.
1. If he somehow got the wrong land feature (Not looking likely since its at links arrow.) or got the right lind feature but made a brainfart and used one other than what he wanted for the calculation.
2. If the overview map is not to scale with the in game map (Possibly to fit it on the game pad, whilst still being functionally usable by human eyes at a 480p res?)
Seems a good size to me for a level 4 LvO game as tightly designed as Zelda. So... assuming we got another 8 dungeon Zelda, that would be 1 dungeon hidden somewhere in every 5 square clicks.
Yeah, I wouldn't take it too seriously as an accurate estimate. But as a rough estimate, it seems quite plausible to me.
I wasn't sure what you were suggesting by field of view though. Which part of his calculation would've been altered by the fov?
As for #2, I assume you mean in proportion to the real map? Yeah, there's no way of knowing that for sure. I think generally the safest assumption is that it is in proportion. The reason being that that's the easiest way for developers to handle drawing the player's cursor on the minimap (just read the X-Z coordinates and then scale them down by a constant)