I think I am a little confused by formulas provided in the original question:
"=nz([Identified needs - Sales],0)+nz([Recommended a solution - Sales],0)+nz([Asked for the sale],0)"
"=nz([Possible 36],0)+nz([Possible 40],0)+nz([Possible 41],0)"
The Nz([expression], 0) will change a null or blank value to 0. So if you have that in your formula it will include all blanks as 0. Another way to handle null values is using IIF(IsNull([expression]),[Value or expression if true],[value or expression if false]) However if you have multiple numbers to calculate this can be a mess of nested IIf statements.
Are you able to upload the database to www.wikisend.com and provide link and password(if applicable). And then point me to the form you are looking for. It is difficult to tell what is going on without seeing it. I understand this may contain sensitive data...if so a work around would be to export the form and applicable tables queirs to another database and delete data or input fake numbers.)
If not can you explain a little better the formulas above. ie: Provide the formula you are using and an example of the values in each field. The example in your question shows the example of 2,0,N/A, Should = 66% Am I correct to assume that this would be 2/3=.66.
If you can't upload the database I will upload one making some assumptions then you can tell me what to change etc until we get it right. Sound Good?