Hi @Wolfgang_Schobel welcome to the community. Since the dashboard tiles are like grid system so the tiles may flow when shown on different sizes of display.
You can reference to the post below for using your browser’s developer tool to mimic the tablet/phone resolution when editing the dashboard on your desktop.
Or you can have your tablet next to you when you edit the dashboard. The updates to the dashboard will be synced to all your devices immediately when you save the changes. So you know how it looks on the particular tablet.