Set GUI to Scale at Constant Rate as Parent?

Reedited it to clarify: I have a shop GUI that has a scaled width. On the left is a banner that has a 210 offset and on the right is a shop header that has a scaled with. I want it to extend from the border of the offset banner to the way edge of the shop GUI, however, it's not scaling at the same rate as the shop GUI is.

Here's a video of it happening: https://i.gyazo.com/587f23ea9d3782da000a9b40829b8f8e.mp4

As seen in the video, the distance to the right border of the shop GUI changes. Is there a way to make them scale at a constant rate either through editing properties (preferably) or through a script?

Thanks for your help :)