I asked this a while ago and several Flashaholics were kind enough to answer. Trouble is, they assumed elementary circuit knowledge that I simply don't have.
I think the forward voltage symbol is vf, as in voltage forward, or very frustrating? What's significant about that figure, what does it actually tell me in practical terms?
In the case of 5W luxeons I thought it was a measure of efficiency, but that appears to be covered by flux because flux tell you how many lumens per watt.
So if anyone can help me to understand this you will make this electronically challenged Flasholic very happy.