I can tell from your posts here that you are experienced enough to know this, so for the benefit of other readers I'll just mention that sometimes the smartest approach is to burn the entire edifice to the ground, let its ashes be dispersed by the four winds and start afresh.

That is exactly our intention (no refactoring in VBA), but the mathematics hidden in the Excel files has to be reproduced exactly.

Some background (as much as I can tell without risking a lot of trouble): Measurement results from several days of running a device on a test rig under varying conditions are written to CSV files, then read into the first Excel file, processed, saved, passed along to next N Excel files, and finally results in a set of calibration parameters that will be written back to the device. The various Excel files could generates a lot of printouts that are already disabled in the code, because nobody cares. But whatever replacement we will build, it must generate exactly the same calibration parameters given the same measurement results. It must be bug-compatible with whatever happens in the Excel files.

And, you may have guessed it: The entire mess is strictly confidential. I can't show the VBA code, I can't explain more details.

Alexander