Working with an IF statement in Excel when the divisor might be -0-

I have numeric values in L34, L35, L36, L37 & L38.

Which generate values in P34...P37, unless L38, L37, etc. is -0-, then N/A is generated.  This works.
But, In R34...R37, the values of P34...P37 are used for calculation.  When the divisor is N/A, obviously I am getting an error.  I want to generate another N/A in the R series cells when the divisor is N/A.
T34..T38 generate %'s based on the values of L34...L38, with L38 being the base year (divisor).  When L38 is -0-, I want the base year to be L37 and on up.  
I have attached a small portion of the spreadsheet with the exact formulas.
Bill GoldenExecutive Managing MemberAsked:
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helpfinderIT ConsultantCommented:
could you attach the sample please?
Bill GoldenExecutive Managing MemberAuthor Commented:
Thought I did.  Let me try again...
ProfessorJimJamMicrosoft Excel ExpertCommented:

in your RO column you can wrap your formula with IFERROR

example R34 like this  =IFERROR(SUM(($P34+P35+P36+P37)/4),"N/A")

to fix the item in Column T then you can use the IF function with offset

so for example in T34 you can put the formula =SUM(($L34-$L38)/IF($L38=0,OFFSET(L38,-1,0,1,1))/4)

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see example

Bill GoldenExecutive Managing MemberAuthor Commented:
Still Testing
Martin LissOlder than dirtCommented:
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