Formula reassurance
 

As I'm new to this I need some reassurance that the following formula I found
in a search will not fail me. The formula seems to do everything I want and
appears to work correctly but I'm unsure of a few things. What I need it to
do is average the last three entries from column BB in the respective row.
The range is X9:BB9 where the formula appears in cell BC9. Here is the
formula:

{=IF(COUNT(X9:BB9)=0,"",AVERAGE(INDEX(A9:BB9,LARGE (COLUMN(A:BB)*(ISNUMBER(A9:BB9)),3)):BB9))}

I only ask because it will not work unless I change the range in parts of
the formula to A9:BB9. If changed to X9 the formula does not work. Note:
A9-W9 are merged cell which contain text. Also note that cell X8 - BB8 also
contain formulas. Since this formula works correctly will it continue to
work correctly in th future.
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Mike Mast
Special Education Preschool Teacher