You can police (or rate limit) inbound port traffic. Don't believe you can actually specify, directly, 50%, but you could, for example, specify that FTP is limited to 500 Mbps on a gig port.

You could also shape egress FTP traffic to fixed percentage (using "shaped mode"). (You can't shape FTP directly, you shape using the egress queues, so if you only place FTP in only one queue, then you directly control it.)

There are some slight capability feature differences between the software versions on the switch, for instance whether you only want to police at the physical port or at the SVI.

See the "Configuring QoS" section of the configuration guide for details, e.g. http://www.cisco.com/en/US/products/hw/switches/ps5023/products_configuration_guide_chapter09186a008081df7e.html#wp1408392

PS:

Instead of restricting FTP's share of the bandwidth, another approach might be to place FTP in an egress queue with a low % allocation using "shared mode". This helps to keep traffic like FTP from adversely impacting other traffic, but allows it to use all available excess bandwidth.