How can I export data from TCPstream Graph and calculate the Throughput

Dear All

I need to create a file that allows me to calculate the throughput of the network and check with the values ​​of the graph TCPStream. How can I export the graph data TCPStream.

I have calculated the throughput considering the following formula:

throughput = packet length / (time of arrival of the packet - time of arrival of the previous packet). Using this formula I have found differents values from the graph of TCPStream

Example: Considering the values below: Observe that the packets are filtered by: ip.addr == 10.136.1.12 and ip.addr == 10.250.40.10 and tcp.port == 102

No        time             source       destination     protocol   length  info
8       0.012479        10.136.1.12.    10.250.40.10    PRES        205     Data Transfer (DT)…
21      0.030623        10.250.40.10    10.136.1.12     TCP          66     iso-tsap > 60049…..
305     0.031452        10.250.40.10    10.136.1.12     PRES         95     Data Transfer (DT)…

throughput = 66 * 8 / ( 0.030623 - 0.012479) = 29100.5291 bits/sec

Please, what is wrong with this way to calculate the throughput