Based on what you are describing, it sounds like the resolution of the devices is slightly different (I thought they were the same, but perhaps there’s something different about how the phone sees the available resolution). Usually in the case of devices with different resolutions, the recommended solution is to design a dashboard that targets each device.
If the devices are close enough in size, you may be able to target a resolution that is close enough between them (as you alluded to), but usually the sizes are different enough that it wont work.
Keep in mind there is a tile size of Custom if you want to try other sizes (like 100px as you mentioned):
