"Dallas is only ~250mi N of Austin, too, and I'm expecting us to get the biggest rainstorm I've ever seen at the very least - we're only about 400-odd from Houston."
Dallas is only 240 miles from Houston. ;)
Austin to Dallas is 196 miles.
City center to city center? Last time I went from D-town Dallas to D-town Austin, I seem to recall it being about 225-250mi.
On the other hand, that *is* road distance. You're probably right about Houston, it just *seems* like 400mi.