Prarie's suggestion of adding a final ELSE is very valid. IF..THEN..ELSE logic should always have a final ELSE in case none of the IF criteria are satisfied. Also, you could try adding some parathensis and switching around your calculation with -1.

MROS_PLAN = IF (((CURRENT_ACTUAL_TP3 - CURRENT_PLAN_TP3)/(CURRENT_ACTUAL_TP8A - CURRENT_PLAN_TP8A)) GT 0 AND
                 (CURRENT_ACTUAL_TP3 - CURRENT_PLAN_TP3) LT 0)
       THEN (-1 * (CURRENT_ACTUAL_TP3 - CURRENT_PLAN_TP3))
       ELSE IF (((CURRENT_ACTUAL_TP3 - CURRENT_PLAN_TP3)/(CURRENT_ACTUAL_TP8A - CURRENT_PLAN_TP8A)) GT 0 AND
                 (CURRENT_ACTUAL_TP3 - CURRENT_PLAN_TP3) GT 0)
       THEN (CURRENT_ACTUAL_TP3 - CURRENT_PLAN_TP3)
       ELSE IF (((CURRENT_ACTUAL_TP3 - CURRENT_PLAN_TP3)/(CURRENT_ACTUAL_TP8A - CURRENT_PLAN_TP8A)) LT 0 AND
                 (CURRENT_ACTUAL_TP3 - CURRENT_PLAN_TP3) LT 0)
       THEN 5
       ELSE ? ;

Please consider the following code where I created your CURRENT* fields (total bogus I might add) then simplified your IF...Then...ELSE. And, OBTW, I did add the ELSE at the end which really is not "required" other then as a "best practice technique".

DEFINE FILE CAR
CURRENT_ACTUAL_TP3   = MPG + HEIGHT;
CURRENT_PLAN_TP3     = HEIGHT ;
CURRENT_ACTUAL_TP8A  = WIDTH ;
CURRENT_PLAN_TP8A    = WIDTH * .95 ;
CALC1/D12.2 = CURRENT_ACTUAL_TP3 - CURRENT_PLAN_TP3 ;
CALC2/D12.2 = CURRENT_ACTUAL_TP8A - CURRENT_PLAN_TP8A ;
MROS_PLAN = IF ((CALC1)/(CALC2)) GT 0 AND (CALC1) LT 0 THEN -CALC1 
       ELSE IF ((CALC1)/(CALC2)) GT 0 AND (CALC1) GT 0 THEN  CALC1 
       ELSE IF ((CALC1)/(CALC2)) LT 0 AND (CALC1) LT 0 THEN  5
       ELSE                                                  0;
END
TABLE FILE CAR
PRINT CALC1 CALC2 MROS_PLAN 
BY MODEL
END

It seems that the source of the error is "(CURRENT_ACTUAL_TP3 - CURRENT_PLAN_TP3)* -1" more then anything else.

I would like you to, also, consider a more structured approach to your coding which would make it easier to analyze and maintain for those who follow (Who knows, It may be one of us).

quote:

(CURRENT_ACTUAL_TP3 - CURRENT_PLAN_TP3)* -1

This will give you error.
Without ELSE phrase, when none of the conditions were true, then WebFOCUS will take the previous true conditions value.

TABLE FILE CAR
SUM
DEALER_COST
BY COUNTRY 
BY CAR
ON TABLE HOLD AS HLD FORMAT ALPHA
END

DEFINE FILE HLD
CC_DEF/A20=IF COUNTRY EQ 'ENGLAND' THEN 'value1' ELSE IF COUNTRY EQ 'FRANCE' THEN 'value2';
END
TABLE FILE HLD
PRINT
CAR
CC_DEF
COMPUTE CC_COMP/A20=IF COUNTRY EQ 'ENGLAND' THEN 'value1' ELSE IF COUNTRY EQ 'FRANCE' THEN 'value2';
BY COUNTRY
END

Follow Dan or Doug method of using IF THEN ELSE.

You might want to focus on Dan's comment about the calculation with the -1.

I have had to enclose a -1 within paranthensis to get calculations to work on numerous occasions.

( (-1) * Field )