If you have "Monthly" Payroll Periods, it will be possible for you to retrieve the value of "Basic Pay" on the last day of the month of June (in the Last Split) and transfer that value into an Information WT. That Information WT will then need to be transfered from Pay to Pay through PCRs. So, if this Payroll Period is for June, the Information WT would be created, but if the Payroll Period is for an other Month the previous value of that WT woud be retrieved and transfered to the current PP.
If you don't have "Monthly" PPs, then you will need a Custom Function or Operation to achieve the creation of the initial Information WT.
That Information WT will need to be created/retrieved before the Bonus WT is processed so that it would be available for the calculation.