Monthly data counter data rollover should be automatically calculated each month

As per title. Currently, I have to manually set the amount of rollover data each month, but it should be an easy calculation for the app to make, for example:

If my plan is for 8GB per month, and in January I only use 1GB. The app should be able to automatically add 7GB to my data plan for February, for a total of 15GB.

This should be a simple toggle somewhere in the Data Plan settings.