Well, here we go again.
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I was feeling facetious, so I asked Google for directions from Nagai to Kisarazu. It does just fine for ground routes, but as far as I know, there's no as-the-crow-flies distance measurement option. Using a ruler, I measured 30-odd kilometers in our world, which makes 330 km in KnI's world. [Addendum: Found the Ruler feature in Google Maps Labs; it measured 33.1 km.]
There was one sticking point in this chapter that I'm still not completely sure about. Isaki actually says "liaison aircraft" where I had him say "air courier." So what's a liaison aircraft? It's a small plane used for non-combat military purposes, like transport and reconnaissance. The Pitts Special is not a liaison aircraft, but the L-4 variant of the Cub (the "Grasshopper") is. So perhaps they were talking about the Cub, but the rest of the conversation doesn't make nearly as much sense that way. I might be wrong, though, so I'll come back to this if necessary.