just as a comment to elaborate on AnAppleSnail's post, the key to using most semiconductor products, and perhaps most products used by engineers, is to carefully and completely read the datasheet for that product.
For instance, as I gaze upon the datasheet (version CLD-DS18 rev 12) for the Cree XP-E LED, I note that it lists on page 5 the forward voltage for the LED at various currents for each of the colors. For some of these currents, there is a typical Vf listed as well as a maximum Vf. The number for the max Vf is useful, since you can design your driver circuit knowing that this is the highest it will be under the given conditions. The more cynical/experienced engineers will ask "hey, what temperature is that data good for?", and as I look at the datasheet, it really doesn't say. I would assume that it only applies to 25C.
Looking elsewhere on page 5, I see that there is a spec for a temperature coefficient of voltage for the different colors. For white, it says there is a typical coefficient of -4mV/deg C. Therefore, if I expect my white LED to operate at 50C, I should expect a change of (50-25)*(-0.004) volts, or -0.1 volts. This puts the typical Vf at 50C at 1A equal to 3.4v instead of the 3.5v at 25C.
Also note on page 8 that there are graphs showing the typical change in Vf as the LED current changes. This is handy for estimating what Vf might be at 400mA, for example, or at least knowing how much the actual Vf will change as current changes.
The reader will also note that the datasheet doesn't list the max Vf for the higher current levels, and doesn't list any min Vf at all! For all you know, the Vf could drop to 2.5v for some random part. Would it?? Probably not, but Cree isn't willing to test all of their parts in order to guarantee it. If you asked the Cree applications engineer nicely, she might give you the values that they usually see, but that's nothing that they will stand behind. If it's important to the design of the driver, the engineer will test a number of samples, and possibly set up a test of the LEDs when they are received at the driver production facility.
So... that's my bit of advice for the day.
Read the datasheet carefully and completely. Learn what it means when things aren't included in the datasheet. Never "assume" that a specification will be what you want it to be just because it isn't in the datasheet.
I think Bob Pease did a nice article on "how to read a datasheet".... I might have to look for it among the columns he wrote for trade magazines.