Calculating time, but with an exception

[bndouglas]

I have a calculation to calculate the elapsed time for multiple measures I am tracking, arrival to x, start to read, etc.
What I want to do is try to add an exception, so that if and only if, the arrival mode listed would be "IPSA", that instead of calculating the measures from the arrival time, they would be calculated from the alert time listed.

Current formula ex: =IFERROR(IF([@[Time.CT.Started]]="",-9999,([@[Time.CT.Started]]-[@[Fml_Arrival_Time]])*1440),-9999)