Myrtle's Bathroom

"It's been out of order all year because she keeps having tantrums and flooding the place."
     -- Hermione (CS8)

Upon his first visit to Myrtle's bathroom during his second year, Harry's opinion was that this was the gloomiest, most depressing bathroom he had ever entered. The bathroom's wooden cubicles are worn and ill-maintained, and even the stone sinks are chipped. Like many of Hogwarts' rooms - including the opulent prefects' bathroom - the room is candle-lit, but the candles are in holders rather than a chandelier and there are not enough of them to light the room properly.

The sinks, which form a row under a large cracked and spotted mirror, have copper taps, one of which has a tiny snake scratched on the side. Apparently that particular tap is a complete dud when considered as part of the plumbing; it serves another function. In fact, if given the proper password, that particular tap will glow and spin briefly before the entire sink drops down out of sight to expose the large pipe leading to the Chamber of Secrets (CS9, (CS16).

Which floor? Differences between the U.K. and U.S. editions
Harry confirmed that Myrtle's bathroom is "three floors below" the prefects' bathroom (which is on the fifth floor), so this room is definitely on the second floor (GF25). So why is this a question?

In both the U.K. and U.S. editions of CS8, Hermione said at the Deathday Party that Myrtle haunts a bathroom on the "first floor". This in itself gives rise to confusion because what in the U.S. is referred to as 'the first floor' is in the U.K. called the 'ground floor,' while the U.S. 'second floor' is in the U.K. 'first floor.' (See the St. Mungo's floor guide in OP for an example.)

However, when Harry, Ron, and Hermione run upstairs following the voice, they run up to the second floor (and CS8 clearly states that they are on the second floor) to where they see the cat hanging outside the bathroom.

In the U.S. hardcover CS8, start with the Deathday Party, which was held in a dungeon.

Harry sprinted up the marble staircase to the first floor, Ron and Hermione clattering behind him.

...ignoring Ron and Hermione's bewildered faces, he ran up the next flight of steps... hurtled around the whole of the second floor...

So it looks as though Hermione's remark in CS8 is an error in the text. It isn't consistent with the other references to the bathroom's location, or with either U.K. usage (which would be 'second floor') or U.S. usage (which would be 'third floor').