Be cautious with this one. People may wish to limit their download and therefore you shouldn't just increase your download
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Imho, the wrong thing is changing the user set limit in the first. This should be done transparently by the software, i.e. it will limit the download speed according to the ratio without changing the user set limit. (So you can set your up limit to 1KB/s, your user set dl limit will remain at for instance 36KB/s while the actual download limit will be 3KB/s according to the ratio. When you increase the ul limit to e.g. 20KB/s, your user set dl limit will apply again.) The user has to be informed of this, of course.