Hi,
I don't know exactly about the question you are asking but i think its very simple to do.
suppose you are having var(i,j,k,l)
so its simply you can do to average about a single point also suppose you are having k=1:27, 1 corresponds to surface and you want to make average of your 5th level only then you have to do like this
let lev5_ave=var[k=5@ave]
this way you can do to average out the level and it works also.
I hope this is the thing you were asking.
regards, saurabh