I made myself a little unit with a socket for the LED pins, a 9v battery clip, and a 12-position rotary switch to select between a bunch of standard resistor values from 1k up to 18k. To find the optimum standard-value current-limiting resistance for an otherwise unmarked LED sitting in your parts bin, just set the switch to 18k, plunk the LED in the socket, and turn until you get the desired luminance. It will let you zero in on something workable for anything from 300mcd up to 3000mcd types. A dozen positions is not enough to cover all bases, but you'll know that if 4k7 is too dark and 3k3 brighter than you really need, you can use 3k6 or 3k9. It's not more precise than using what Luciferstrip suggests, but the resistance values are pre-identified so you don't have to drag the meter out to measure.