Dynamic setting of custom vars?

[Greg Fenton]

I am trying to set som custom vars based on conditions of the 
system we are running on.

In my .emacs, I have the following code:

;-----------------------------------------------------------
(cond
 ((string-match "my-laptop" (downcase (system-name)))
    (setq glf-height 60))
 ((string-match "my-linux-box" (downcase (system-name)))
    (setq glf-height 80)))

(custom-set-variables
 '(default-frame-alist
     (quote ((width . 81) (height . glf-height))))
 '(initial-frame-alist
     (quote ((width . 81) (height . glf-height))))
)
;-----------------------------------------------------------


However, it appears that the value of glf-height is being ignored 
during the creation of the intial frame (trying to create 
subsequent frames throws an error "(wrong-type-argument integerp 
glf-height)".

I assume I must reference the variable in some other way in the 
custom-set-variables or that the value of glf-height is not 
visible to the make-frame-command.

Is what I'm doing at all possible?  Is there a better way (should 
I simply avoid custom-set-variables)?

Thanks in advance,
greg.fenton