Get Total Waiting Time of a request

Summary

Get Total Waiting Time of a request for SLA calculation

Content

Hi, 

Need help calculating the Total Waiting Time of a Request for SLA calculation.

Total Waiting Time = The total time in which the request is in Waiting status state.

I am trying to do it using inc_performance table using the formula - sum(if(inc_performance.intv_type = 4,inc_performance.rel_time)) - sum(if(inc_performance.intv_type =16,inc_performance.rel_time))

But this doesn't give the correct results in some complicated business workflows like, various status changes (when the request status is set solved more than one time).