It's just personal preference. I like it 2/3 of the way up in bathrooms, and 1/3 of the way up in the rest of the house. If you split a sheet of panelling into thirds, that's the height for the shorter panelling (you'll get three widths per sheet). If you use 2/3 of the sheet, that's for the higher panelling, and you'll have the 1/3 left over to do another room in the shorter height.

Personal preference, really; if you absolutely must have a hard number, try 48" or 54" but it's just as likely that neither will fit your room just right. :-)

There are a lot of things you can use as a reference point for the cap rail topping the wainscot (to be obnoxious and persnickety, it's only a "chair rail" when it is in a living area and placed at a height to keep a chair from banging the wall): a windowsill, the top of the toilet, the top of a vanity backsplash, a medicine cabinet frame or window frame (1/3 of the way up a window frame or medicine cabinet frame is a very classic relationship), above or below wall outlets/switches. Tape a string up around the wall at different heights to get an idea of which one works for you. Consider, also, how your baseboard is going to work - if you are going to put the beadboard on top of a baseboard, behind it sitting on the floor, behind it but raised above the floor (use scraps of beadboard to fill the dead space behind the baseboard). This will affect your finished height.

Sheer practicality and avoidance of waste is a pretty important thing to consider too, so if you can use a half or a third of a plank/sheet instead of, say, three-fifths (leaving two-fifths to waste, unless you have a use for it elsewhere), that's better. It is often more cost-effective to get the full size planks/sheets and cut them than to buy the smaller "wainscot size" ones.

In my previous house's master bath, I went for 2/3 of the way up the walls, allowing us to use half-lengths of the 8' long beadboard planks to minimize waste: 48" tall beadboard on top of 5.5" baseboard (seam disguised with an inexpensive trim piece called a "base cap") and topped with 1.5" cap rail = 55"; exactly 2/3 would have been 60" but the beautiful wallpaper border that went above the wainscot brought the total up to just the tiniest tad under 61". I had originally wanted to use 1/3 up the window frame for the cap rail's reference line but the beadboard planks would have had to be about 54", which would have doubled the amount I would have needed to purchase!

We recently renovated a very small full bathroom at our lake cabin. We decided to go about 2/3 up with the beadboard, because to do it halfway would have really chopped the room in half visually and made it appear even smaller. Also, I had the contractor add a 4" wide board as a cap on top of the beadboard and then finish this board with a thin ledge. We then installed big towel/robe hooks (really nice polished nickel ones that I got a local discounter for .99 ea) down one entire wall. Since this is a vacation home and we tend to get lots of guests and also rent it out, these hooks are much more preferable to towel racks. I then painted the upper portion of the wall a nice sagey green and put up some art. Looks great. I'll try to find a picture when I get home from work later and post it.

I situations like this I always stick with the Rule of the Golden Mean. In a nutshell, this rule essentially says that a ratio of 5:3 is most pleasing to the eye. This rule has been recognized since ancient times and was utilized even when building the great structures of Rome and Greece.

Therefore in a room with 8' ceilings I have the beadboard installed up to 5 feet.

Nicole

Thanks everyone for the helpful suggestions.