Coordination of Benefits? New job late in year – how does deductible work?

Me – FTE with a Premera Health plan. We’ve met our deductible for 2023 and are well into co-insurance (near max OOP for a family)

Wife- was SAHM (unemployed) listed as a dependent under my Premera plan

Life event: Wife went back to work this fall, and will have a new health plan as Primary with my Premera health plan as secondary retroactive 9/1.

Question: Wife’s new plan has a $2k deductible, $5k out out of pocket max for an individual (coinsurance covered at 80%). Lets say given her current healthcare visits I expect her to get charged $5k in healthcare bills between 9/1 and end of year. If she stayed on my plan, my out of pocket would be $500 (90% of that $5k would be covered by my plan)

Given she is moving to new plan – how much should I expect to pay of this anticipated $5k in bills? I know if she only had a primary plan (ignoring my plan as secondary), it would be the full $2k deductible then 20% of $3k ($600 in coinsurance) so $2600…but I know coordination of benefits should make this total out of pocket cost be less…since her secondary insurance (My plan) is already beyond deductible. I don’t know how to calculate this…any help would be great!