How to handle define and EQU ?

I want to write some useful definitions in my source and have a problem resolving EQU's.

I want to make some EQU in my source and check them with "if defined" but don't have success.

Here my definition:

Code:

MEM_1K:

if defined MEM_1K
   MEMAVL  EQU     1024
end if
if defined MEM_2K
 MEMAVL  EQU     2048
end if
if defined MEM_4K
 MEMAVL  EQU     4096
end if
if defined MEM_8K
 MEMAVL  EQU     8192
end if

This does not work. I don't know why.
I always get MEMAVL=8192 when defined the label MEM_1K first.
Anyway there is no define at all for MEM_8K.

So how can this be handled in an easy way ?
I just want to define the available memory at start of program.
And there is a check done at end of the program which works fine with assert (when MEMAVL set correctly).

Code:

assert ($-MEMST)<MEMAVL

I just want to have a definition of available/allowed config fields.
I don't like to write simply MEMAVL EQU 32768 for example.
Is there a chance to get this under fasm ? Very Happy