have you seen this thread where I did some tests??
http://www.dslrusers.net/viewtopic.php?t=11579
basically they aint kidding about burning your flash. or your lenshood either if you get the sun the other way. I would recommend not walking around with the fresnel lens fitted.
I think it is worth knowing where it is firing and you may have to add velcro etc to get the beam correct.
I set up a test with my 400 mm lens set at 80mm where I could see a bright spot centre of the image and adjust the shims/ beamer position on the flash til I got it centred.
you could also use the
modellng flash function on your flash and use the mk1 eyeball to decide alignment.
max range is achieved with the flash set @ 28mm but you would need to assure that it was centred correctly to do that. the 50mm recommendation allows for some misalignment. test it and see.
Steve