If direct changes are made to the plan rows, then the whole link to a usage and a price will be broken and we will not be able to prepopulate future plans correctly.

The best practice is to change the unit price, if possible. The prices initially shown are estimated using historical information, and are the most likely source of  any errors.

If you know the total cost for the month, then divide that by the expected usage shown to find the unit price that will give the value you want. The diagram below should help with how the formula shown at the top of the page can be used to work out other values.