Setup to prorate pre tax deductions - on all OAB elements

I need to setup a proration on a pre-tax benefit deductions elements whenever there is a change on assignment like new hire or Term, (also anytime a employee drops or changes his benefits election).

I have 2 payrolls, weekly and bi weekly.

How can I do this ? If someone already have done this type of requirement, please share notes or documentation or steps that will be helpful.

Any known issues, that need to be taken into consideration while doing this requirement -will also be helpful.

Thanks,

Ruhulla