Trev,
Assuming your slider is ranged from 0 to 100, the stroke parameter can be directly linked via the pick-whip. But the position parameter is going to require the “linear” command.
Now of course, position has two values – x and y – so understand in the expression you will need to define two parameters, one for x and one for y.
So let’s say the layer only moves left to right along the x axis from a value of 40 to a value of 600. The y position remains unchanged. You can write this expression for the position.
var amount= [HERE YOU PICK WHIP THE SLIDER CONTROL VALUE];
var x=linear(amount,0,100,40,600);
var y=transform.position[1];
[x,y]