Calculate cost of subscription based on date of order

I have a reference table named "Holidays" that has date column "NewOrderDeadline" and two number columns "FirstYear1Flag" and FirstYear2Flags".

I am create a form to enter a record in "NewOrders" where I ask how many flags in a field called "FlagCount".

When they submit, I am also updating the "OrderDate" field to be the current date. After submit, I open a child pane to edit the record. On the second screen, I have a "cost" field that I want to update by looking up the orderdate < = NewOrderDeadline and FlagCount = either 1 or 2 to get value from either FirstYear1Flag or FirstYear2Flags fields.

This is a pretty simple look up but I can't work out how to do it in knack. I know JS is an option, but my goal is to eventually hand this off to a less technical volunteer to maintain in the future.

Any/all suggestions are welcome!