 Using the -W switch
You don't.
The RTMPDUMP program contains the switch -W which does that calculation for you automatically.
And all the parameters outputted by the -W switch are saved automatically in a file named .SWFINFO in your RTMPDUMP directory.
The information distibuted with the standard release versions of RTMPDUMP explains what url to specify in the -W switch, using this format:
-W "http://{URL}"
Here's an example -
-W "http://www.bbc.co.uk/emp/9player.swf?revision=18182_20020"
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