SSAS dealing with non linked data

I'm trying to compare forecast data with sales data in a cube. Sometimes we have a forecast with no actual sale, sometimes we have a sale with no forecast. How do just show 0% if one or the other does not exist? This is my current code... Green lines are ok because it is showing 0% where one of the amounts doesn't exist. I want it to do the same for the red circled ones.

CREATE MEMBER CURRENTCUBE.[Measures].[Forecast Accuracy Amount %]
 AS IIF([Measures].[Sales Line Amount TCUSD SF] <> 0 OR [Measures].[F Amount] <> 0, 1-ABS(DIVIDE(([Measures].[F Amount] - [Measures].[Sales Line Amount TCUSD SF]) , [Measures].[F Amount])), NULL),
FORMAT_STRING = "Percent", 
VISIBLE = 1 ,  ASSOCIATED_MEASURE_GROUP = 'Sales Forecast'  ; 

EDIT - if both measure values don't exist they would both be null and wouldn't display. So I wouldn't want the percentage to display either and I only want to do the calculation if one of the quantities is not 0. Forecast accuracy should not show 100% if f amount is 0 and there is a sale as shown in the last img... this is the last thing to correct...

I prefer the safety of ISEMPTY:

 CREATE MEMBER CURRENTCUBE.[Measures].[Forecast Accuracy Amount %]
 AS 
 IIF(
     NOT ISEMPTY([Measures].[Sales Line Amount TCUSD SF]) 
     OR 
     NOT ISEMPTY([Measures].[F Amount])
   , 1-ABS(DIVIDE(
             [Measures].[F Amount] - [Measures].[Sales Line Amount TCUSD SF]
            ,[Measures].[F Amount]
            )
          )
   ,NULL
  ),
FORMAT_STRING = "Percent", 
VISIBLE = 1 ,  ASSOCIATED_MEASURE_GROUP = 'Sales Forecast' ; 

Slightly more complicated:

 CREATE MEMBER CURRENTCUBE.[Measures].[Forecast Accuracy Amount %]
 AS 
 IIF(
     ISEMPTY([Measures].[Sales Line Amount TCUSD SF]) 
     AND 
     ISEMPTY([Measures].[F Amount])
   , NULL   //<<if both are empty
   , IIF(
       NOT ISEMPTY([Measures].[Sales Line Amount TCUSD SF]) 
       OR 
       NOT ISEMPTY([Measures].[F Amount])
      ,1-ABS(
          DIVIDE(
            [Measures].[F Amount] - [Measures].[Sales Line Amount TCUSD SF]
            ,[Measures].[F Amount]
          )
         )
      ,0
      )
   ),
FORMAT_STRING = "Percent", 
VISIBLE = 1 ,  ASSOCIATED_MEASURE_GROUP = 'Sales Forecast' ;