The confidence in a mean and SD when n30 (in relation to the population
values) is quite low compared with n=30.

Doesn't the variability come into it as well as the population size.
My recollection (without looking it up so it might be wrong!) is that the
confidence level is derived using Student's t values from tables. These vary
with population size but after about n=10 the value for 95% confidence (most
commonly used) is approx = mean +/- 2* SD.

Dave