PS....

I wrote:
If the mean is 6 and the std dev is 4, the normal distribution
curve will become negative to the left of -1.5sd, i.e. when
RAND() is less than NORMSDIST(-1.5) -- about 6.68%.

I should have noted that -1.5sd depends on the mean and std dev. More
generally, the normal distribution becomes negative to the left of -mean/sd;
e.g. -6/4.