Ok. First of all, regarding your “light tracks”:
If you want to manipulate the paint strokes writing on, you’re NOT going to want to use a shape style from the light category. Often times, those styles have animated properties in the color and opacity that will be contrary to what you’re looking to do, and extremely difficult to manipulate how you want. You’re better off creating the effect from scratch so you’re in control of everything.
Secondly, I wouldn’t bother with the write on behavior for this particular effect. The write on behavior manipulates the First/Last Point offset. Since you need to keyframe the write on, I would just use the last point offset parameter to animate on. Fewer moving parts. Your previous problem with last point offset was likely due to a shape style. Building the light track from scratch will eliminate that problem.
However, to check your problem with the Custom Speed setting in a write on effect, I find it helpful to open the keyframe editor so I can see what’s going on. You should be able to see the animation line of the Last point offset. Then if you apply keyframes to the custom speed parameter, you should be able to see it working properly or not in the editor. But that’s more of an aside. As I said, I would skip the Write On behavior, and just keyframe the Last Point Offset.
Andy
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