Implementing the 415c limits for a new after-tax non-roth 401k plan option for 2025?

We are setting up a new non-roth after tax contribution for a 401k plan. We recently added the enhanced catch-up limits for employees age 60-63 for the 2025 calendar year (not subject to Roth income testing requirements until 2026). It's unclear if employer contributions to the 403b plan need to be added to the 'deductions subject to the limit' on setup of the 401K 415 c plan limits. Have you implemented this type of plan option for the 401k and what were some of your challenges?