Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
pro rata [prəuˈrɑːtə] adv пропорциона́льно
adj пропорциона́льный*пропорциона́лен
on a pro rata basis на пропорциона́льной осно́ве
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.