I also use Transmission, but I like to seed for a while after download. I keep the defaults of x2, but I often quit before the upload had finished.
If you want to quit when finished downloading then there is an option to call a script when a download is completed. You could shutdown Transmission using this. But please think if this is what you want to do. Why not set it up so that you at least upload the same volume you download (x1)?
Desktop: Dell, Intel Core 2 Duo E7500 @ 2.93GHz, 2GiB RAM, Lubuntu 14.04 (trusty)
Notebook: HP 14 Notebook PC 14-r003TX, i5-4210U @ 1.7 GHz, 8GB RAM, Linux Mint 17.1
Notebook: DELL latitude E5520, i5-2520M @ 2.50 GHz, 4GB RAM, Linux Mint 17.1