ffman22 sorry I thought I had already replied to this. It seems to be working for me.
The quick test I did was:
Every Tick - Set current time in animation to unlerp(viewportLeft(0),ViewportRight(0),Mouse.X(0)) ratio of total length.
Then the left side of the screen is the beginning of the animation and the right side of the screen is the end, and wherever your mouse is in between is what the animation should play back at.