There are so many facets to this, more than just latency between continents. The location, speed, and load of the email server Bro/Clack is using is probably the major factor, and most of that isn't terribly predictable. You can do a tracert to see how long it takes to hop through the different nodes, but it's not the same protocol as email so it won't be exact. Still, could give you a good idea. Hope that helps.
Edit: one more thought; you also have to pay attention to the physical and virtual location of your own email server. For example, if your email is with gmail, it could get spit out from a number of Google servers around the country, and even around the world.
