Bandwidth is always communicated in BITS per second, not BYTES per second. The main difference (if properly communicated) is whether the B is a big "B" (for bytes) or a little "b" (for bits).
So if you are buying a 20/20 circuit, I am sure it is 20 megabits per second, or Mbps, not 20 megabytes per second, or MB/s. Since there are 1024 Kb per Mb, you need to take 1024 x 20 to find the Kbps, so on the FortiGate you will want to set the bandwidth as 20480 Kbps.
I hope that helps clarify for you.
- Daniel Hamilton
P.S. storage is usually communicated as bytes, while bandwidth is communicated as bits. This is helpful to keep in mind...a 20 megabyte (MB) file will not transfer in 1 second over your 20 megabit (Mb) WAN, but will take more than 8 seconds (due to 8 bits in a byte). A good rule of thumb for bandwidth vs throughput is to assume a 10 to 1 ratio to allow for overhead. So 10MB will transfer in approximately 1 second over a 100Mbps link.