I don't know if your field are extracted
if the field are extracted you only need a eval condition
eval "Total time taken"=Total_Time_Taken/ No_Of_Requests
if you need to extract the fields before needs a rex comand
| rex field=_raw "No_Of_Requests=(?<no_of_requests>\d+),Total_Time_Taken=(?<total_time_taken>\d+)" | eval "Total time taken"=Total_Time_Taken/ No_Of_Requests
anyway I suggest to extract the field with an automatic KEy-value extraction