Does anyone have a weighted average formula (or any other formula) you use when determining earnings for a QDRO split?

I go fund-by-fund, transaction-by-transaction, and do an exact calculation, tracking the Alternate Payee's gain/loss within the Participant's account.

Then, to ensure that I didn't make a major error, I apply my reasonableness test to see if the Alternate Payee's current balance is within the ballpark.

The value of the Alternate Payee's award at the time of the split is 1.
I take the Participant's balance at the time of the account split and add half of contributions that have come in since the split. 2.
I divide 1 by 2 and multiply that times the dollar gain/loss in the participant's account from the split date through today, when we are actually splitting the account. The result is usually within a de minimis amount difference from the exact calculation.

[This formula assumes that contributions come in on a relatively even chronological basis]