Calculates value using two different fields

Have seen this asked before but can’t seem to find a clear answer to the question. Trying to set calculate field based on two form fields

- Dept is a calculated field, result of lookup based on current user

- Institution is a multi-select lookup control

Will need to lookup another list based on combination of both, and concatenate the resulting values into a calculated field.

Institution    Dept       CostCentre

hosp1          Finance  A1

hosp2          Ops        B2

hosp1          Ops        A2

....

If both hosp1 and hosp2 selected and Dept is finance, then resulting value of calculated field is “A2, B2”

Previous threads sometimes imply this can be done or can’t be done. Im using onprem 2013. Don’t mind using Javascript if necessary, preferably not but don’t mind if no choice.

Can’t use a helper column as per this thread because SharePoint doesn’t allow lookups to be part of the formula for calc field.

https://community.nintex.com/t5/Nintex-for-SharePoint-Forum/Nintex-Forms-set-Lookup-value-using-javascript/m-p/108270

Any guidance on this? Thanks so much, I’ve been searching the community for hours.