Rule of thumb for LED Ballast Resistor
This is something that is dependant on the LED and power source, as well as the operating conditions. If you want a cover-all rule of thumb, assume at least 0.5V, or go if you need absolute reliability for 1V.

Of course, if you want absolute assurance you are not wasting too much energy and still not damaging the LED, you can:


Get all the datasheets together, inspect the LED's power curves at all the possible temperatures, then compare that to whatever voltage regulation you intend to apply, choose the worst-case scenario and model your resistor for that, accepting that in the best case the current may drop noticably.
Design a switching current-mode driver. There are a million and one LED driver chips in the selections of Linear Technology and it's companion companies (Probably TI and AD have some as well, for example) that drive a LED through switching a coil while monitoring the LED current with a small sense resistor, often only "wasting" 100mV or even less.