Understanding Offset in Effects Engine

I'm trying to create a "comet" effect for a string of Color Kinetics iColor Flex. I have 50 units on a string, and I'd like to have a comet run through the string and then repeat. I have read the manual, and it sounds like using a Sine or RampInv effect is the right effect, but I've been trying to fan the offset to create the motion and can't seem to find the right setting. Any ideas?

Thanks
Dan
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