Your formula calculates the difference in workdays for “Issue” dimension members at issue level as you are using two properties for issues. But the same formula would not work if you don’t use “Issue” level in the report.
To calculate results on month level (aggregated from issue results), you would need a formula that iterates through set of filtered issues (for each month) and then return the % results for SLA calculation.
Try this formula when creating a user-defined measure with % format
[Measures].[Issues closed count]>0
[Measures].[Issues closed count]
Martins / eazyBI support