The Mac keyboard layout is weird. Some choices Apple made when designing it are good but some of them really come from outer space. Why would anybody need a ± symbol on his keyboard is beyond me. Luckily Ukelele software provides an acceptable solution.
Personally I use U.S. Extended layout for almost everything. The only time I switch is when I have a very lengthy french text to write. (Speaking of which, why does french layout have a character directly on the keyboard? Just how often would anybody use that?). So, in order to keep the layout consistent with other keyboards I have made a U.S. Extended+ version which changes flips the §± and `~ keys around.
Download it here: