Hi Mark,
I have a similar task, but my "key" is a continuous data type.
In Excel it is very easy to create a look-up table and use it to transform highly un-linear characteristics.
In this case it is battery voltage data to estimates of state-of-charge (SoC) and vice versa. So, I would like to convert e.g. a data table with measured voltage data to SoC estimates. One challenge is that my look-up table may/will not have all the many voltage levels existing in the data, so I need the SoC corresponding to the nearest voltage level. E.g. voltage data is 3.801 V and look-up table has 3.800 V as the nearest and a corresponding SoC of 51%.
Sometimes my task is to convert "backwards" (SoC => voltage), but hopefully I could create that solution on my own given good inspiration
BR, Kim.