If you're finding it difficult to use rtmpsrv.exe and rtmpsuck.exe from the command line, I recommend NirSoft's RTMPDumpHelper:
RTMPDumpHelper is a small utility that can help you to easily download RTMP video/audio streams. By combining this utility and the proxy server of RTMPDump toolkit, you can simply open a Web page containing RTMP video stream in your favorite Web browser, and while watching the video, it'll be saved to your disk automatically as .flv or .mp4 file.
I've successfully used it to record complete RTMP streams, but never tried a partial download so far. If the underlying rtmpsrv.exe/rtmpsuck.exe proxies support it though then what you want to do should be possible.