That site may in some cases be slightly inaccurate.
The KA1's resolution is 1404x1872, which makes its AR 0.75, like almost every other Kobo out there (weird viewport quirks aside, which, surprise, includes the H2O, whose top 11 pixels are hidden, meaning it ends up being 1080x1429 instead of 1080x1440).
At most, it's going to skew off that 0.75 AR by a few pixels for some odd ducks (Aura HD, possibly Glo HD).
So, what you want to do depends on your preference. I tend to prefer honoring the *cover art's* AR, instead of the screen's (i.e., letterboxing on a black background), so I just need to know the screen resolution.
For more technical details, see my answers in
this recent thread.