confidence interval of a average...

I have a sample of lung capacities from a population measured against 
height.  I need to know the 95% CI of the lung capacity of a person of 
average height.

I have fitted a regression line.

How do I get a minimum and maximum values of the 95% CI?

My thinking was that this has something to do with covariance, but how?

My other thinking was that I could derive the 0.975 (sqrt 0.95) CI for the 
height.  Then I could take the lower height 0.975 CI value and calculate 
from that the lower 0.975 value from the lung capacity. And then do the same 
for the taller people.  That is bound to be wrong though.

Dunc