To compute prorated bill, you need the specific date of installation/activation of your account.
It will be computed this way:
MRC divided by number of days within the account was installed/activated, then, multiplied by the number of days from the installation/activation date up to the end of the month.
Example: (FIBERX 1500 25MBPS, date installed: 06 Jan. 2020)
(1500 MRC / 31 days within January * 26 number of days)