Just to let you know after the discussion, the old method of find the equation using ECO can still be done on the latest version of Quartus.
In Quartus Pro ECO is supported primarily only as a Tcl interface. There is some documentation on this here:
https://www.intel.com/content/www/us/en/docs/programmable/683873/current/an-922-using-the-eco-compilation-flow.html (AN 922, see also section 1.3.1) and here:
https://www.intel.com/content/www/us/en/docs/programmable/683641/24-3/using-the-eco-compilation-flow.html (Documentation in Quartus User Guide) and here:
https://www.intel.com/content/www/us/en/docs/programmable/683432/24-3/tcl_pkg_eco_ver_1-0_cmd_modify_lutmask.html (Tcl API documentation)
An easier way to get comfortable with this mechanism is to use the Fitter Toolkit, which is an executable shipped with recent versions of Quartus but only accessible via command-line.
The command to run Fitter Toolkit is quartus_ftk.
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