Occasionally I've told people to go more than 180 degrees, but I usually specify long way around (that's how we do it for aircraft in the navy... example: Let's say someone is going 000 (north) and you want him to go to a course of 150 but for whatever reason you want him to go left, the way I would do it is: "Speedbird 114, turn left heading 150 long way around" so there's no confusion and the pilot knows I didn't really mean right...
Is that how FAA or Nav Canada does it as well?
As for QNH davolijj is correct... it is the barometric pressure (in inches of Mercury) and changes constantly with weather... standard is 29.92, which is what aircraft use if they don't know or if they are above 18,000 feet. Lower than that, it is very important to give the right setting and to ensure all aircraft are on the same setting... even minor changes make big differences in altitude (most aircraft use barometric altimeters, which measure your altimeter by comparing the air pressure outside with what the air pressure is at sea level... so when you say altimeter 29.75, what you're doing is saying based on the pressure at your airport, sea level pressure would be 29.75...). Most people don't realize how important this is but a difference of one decimal point (ie. btw 29.90 and 29.80) is equivalent to about 100 feet of altitude... so you can imagine how messy that can be if you're using the wrong setting...