To calculate BACK-BACK time the test records the time (in ticks) when the last byte (the check byte) was received in the first BACK (call that BACK1).
Then the test records the time when the first byte (the preamble byte) was received in the second BACK (call that BACK2).
The test then performs the following calculation BACK2 - BACK1 == TotalTime. If the TotalTime (the BACK - BACK Time) is greater than RT1, set to 711 ticks, failure point 841 is issued.
To view all the HART packet timings, process the .OUT file with conv tool and record all of the data to a text file. This will aid you with debugging this test case. Documentation on conv can be found in the HART Device Testing Tools User Guide.