Responsive Design / Custom Breakpoints

It would be super valuable if the preview widths you can set in Replo are their own unique viewport where changes made at that size and down do not effect any sizes that are larger. For example, a change at 1200px would not effect 1400px but it would effect anything 1200px and lower. I have noticed that the three default device sizes are not enough in some circumstances to ensure a fully responsive design across every width.