I have three applets designed in VB6 a decade ago. They interface with some instruments (one via serial port; two via ADC card) and provide an on-screen display of an instrument's reading along with other information. The applets make the data available through DDE links and we supply a sample Excel file to show how to access the data and convert that (through a function or look up table) into the value the customer needs. The customer provides the conversion function. (The instruments are photometers, measuring absorbance; the conversion is the inverse of absorbance = f(x) where x is the independent variable.)
It's time to update these and I am looking at C# (each program is less than 1000 LOC; conversion should not be difficult). I know DDE is deprecated; I've been researching the alternatives but I'm not clear what will be easiest for our customers to implement and still be flexible. For example, while we supply a sample Excel [spreadsheet] a customer might want to [use] a different spreadsheet program or a database; hence, creating an Excel add-in may not be the way to go.
Any recommendations on how to proceed? Links to resources are appreciated. If I decide to expand these applets to be a database of the readings what controls [or library package/framework] are available to allow the user to enter their own formula?
Edits in  for clarity