415 excess

We administer a 401K that allows for employee after tax contriiubtions.

The client over-shot 415 by $18K.  Since voluntary, will remove from the plan, but be taxed on the earnings.

In the past, we have used the VFCP calculator to calculate interest from the date of payment to the plan through the date the funds are removed from the plan.

Is this still allowed; or if not, what is the generally accepted method?

Is it that hard to use the participant's individual rate of return?  Or even the plan's rate of return?  Will the record keeper not calculate that for you?

Recordkeeper says TPA responsibility.  Plan investments with Mass Mutual, individual accounts,  about 6-7 funds chosen for all subaccounts, elective, SHNE, Roth and voluntary.