I have some LED shortener boards I bought a while back and I never could really wrap my head around using them. I finally have a good project to use them on to shorten some left over 50 count warm white full wave LEDs from 50 to 25. I need to drop it really to 20 but I figure half would be easier and just tape over the extra lights.

Is there a guide on figuring out what resistors I need to use in the board to shorten the string like that? I can't get a good grasp on it from what I've read so far.

Thank you,