Yes, you will need to cut the end off of the white plastic throttle tube. It is best to trim it off such that approximately 1/8" of the handlebar protrudes beyond the trimmed end of the plastic throttle tube.
I have successfully used a tubing cutter to trim this throttle tube and also a Dremel tool with a cutting wheel.
Your idea of using a light to see the end of the handlebar is a good one. It might save you from having to make two cuts (which I have had to do several times).