[gradsusr] Min max from defined variable

[Pascal Grandjean - F5LEN]

Dear All,
I'm using a script to compute and mapping refractive index
for 4 layers from 950mb to 850 mb.
Variables rh, tmp... used for calculation come from GFS model.
In my script I have:
'define N1=(77.6/T)*(lev+4810*(e/T))'
So I have 4 defined variables N1, N2, N3 and N4.
I can map N1, N2... but with this type of variable
I can't extract max or min values directly with a z variation.
How can I proceed to mapping max or min?
Is it possible to make a new grid file with a new variable
N with 4 levels?

Thanks for any help.

Regards,

Pascal