Hi Abhilasha,
we generally use BADI functionality for these benefits proration.
So lets say the bi-weekly period is 06/02 to 06/15 and cost of plan is $10 and in the mid period i.e. on 06/05 the dependent got added to the same plan then in payroll the record will prorate using BADI as -
EE 06/02 - 06/04 = $2.14-
EE+Depd 06/05 - 06/15 = $7.86-
Hope this helps.
Ashish