join - Call center Calculating monthly and annual percentage -

i'm trying put management information call center.

i've got following info on every call gets logged: [createdon] = date & time of creation [case_status_name] = reason closing call (answered 1st line, transferred 2nd, call request etc.)

now i'm looking make graph show in 1 line % of calls answered 1st line , annual average% of calls closed first line during last 12 months.

i can months [createdon] extract function, same goes year. can group [case_status_name] in 1st line answered , else simple if statement. i'm having trouble getting percentages right both month , year together.

i can sensible data if calculate yearly average or if calcluate monthly average. when try them together, wind getting kinds of crazy values.

what proper way monthly , annual percentage in same table?

1. create query

2. add column [month] expression

   _first_of_month([createdon]) 

3. add query item [total] expression

   1 

4. add query item [answered 1st line] expression

   case when [case_status_name] = 'answered 1st line' 1 else 0 end 

5. add query item [closed first line] expression

   case when [case_status_name] = 'closed first line' 1 else 0 end 

6. filter query

   [createdon] between _first_of_month(_add_months(current_date;-12))                 , _last_of_month(_add_months(current_date;-1)) 

7. add query item [% of calls answered 1st line] expression

   [answered 1st line] / [total] 

8. add query item [12-month average] expression

   sum([closed first line] report) / sum([total] report) 

9. build graph based on query