You're right. The switch calculates the distance in base to the echo delay measured in the ISL.  If you configure the ISL port as LD, with command portshow you'll see the stimated distance (with portbuffershow you'll see the BB credits reserved as well). Also the switch will provision BB credits for that distance, but it is a best-practice to manually calculate the real BB credit needs, because switch calculations can understimate the real bb credit needs.
I have 2 Brocade DCX4S's connected over 1 ISL and I'm curious how much data per second I can send to the other site. I used the commands spinfab and fcping to determine if the ISL is ok and what the latency is.  - spinfab returns that the ISL is fine - fcping returns an average of 4.6 ms  The SFP's used are 4Gb long wave 4 km and we're using DWDM to span the distance. The 2 sites are 42 and 62 km appart.  We are told that the active components of the DWDM will take care of the buffer credits and getting the data accross.  What do you think of this set up and what can we expect in the MB per second?
How can I measure the max speed a dark fiber will eventuall support?