The 20 GB for $20 Data Plus option is only available on the $28 /mo Base Plan and $65 /mo +Phone Plan.
Whether you activated the Plus option at the beginning or at the end of the month, you’ll always get 20 GB in full. Consequently, the $20 subscription charges will not be pro-rated in any case.
Similarly, if you deactivate the Plus option in the middle of the month, you’ll still benefit from the Data provided by the Plus option until the 1st of the next month, because you'll have already paid for it in full.
For more details on the 20 GB for $20 Plus option billing policy, you can check this FAQ.