Do you have some supporting images for that requirement Dave? At first glance, it would seem you would also need to take the scale values in to account to determine the width of the slider control when placing child images. Or simply scaling the images and then just using the slider measurements.
The Canvas Scaler does do very different things based on the UI Scale mode is set to, so you could also try using a different mode.
Simon (Darkside) Jackson
@SimonDarksideJ