help with a tricky expression

https://imgur.com/oZ1fLSu

I’m trying to use the “trace path” function to get a stylized draw-on effect, but I need the pencil to stay perfectly centered. So, the issue is the trace path expression advances along the path using a 0-100 Progress slider. When I animate the position of the path, it’s by pixels. It’s not a 1:1 value. So, I can tweak the trace path expression so the X is always 0. Then it stays centered but doesn’t stick to the path as it animates. Any suggestions on how to get the pencil to both stay centered while also tracing on the line?