"PORTING C" > How to assign a value to a define statement?

Hello,

This question is geared more towards seeking a voice of programming
experience.

I am running into a situation where I need some advice on how to do things.
Perhaps I am seeing this the wrong way. In summary, from main(), I would like
to call a defined macro called "Config_delay_timers", and in that macro I
would like to do some calculations where the result can be assigned to the
value of another #define statement.

Please view the following code:
==========================main.h

#ifndef TEST_H
#define TEST_H

// *** SYSTEM OSCILLATION *** //
#define Config_delay_timers(extCrys, fpllidiv, fpllmul, fpllodiv) \
    FREQ = (((float)(extCrys/fpllidiv)*(float)(fpllmul/fpllodiv))*1000000)

#define CURR_SYS_OSC FREQ
#define OSC_PERIOD (1.0/CURR_SYS_OSC)

#endif // TEST_H //

=====================================main.c
#include <stdio.h>
#include "test.h"

int main()
{

Config_delay_timers(8, 2, 21, 8);
//SYSTEMConfig(CURR_SYS_OSC, SYS_CFG_ALL);

return 0;
}
===================================

I am getting an error where the compiler complains about FREQ is undefclared.
So how is one supposed to carry out a particular calculation via a define
macro and then further assign this result to other define variables in the
program.

I am thinking of mayby using #ifdef preprocessor command but I don't see how
this would be of any use in this particular case.
 
All help or reccomendations is very welcome and apprciated. Thanks in advance!

--
Best regards
Roberto