Hi Craig,

Am I missing anything?

If you're using Excel XP or more recent, 2c is no longer an issue, as
you can instal your COM DLL as an 'Automation Addin', using Tools
Addins Automation Addins.

Option 4 is to write the addin using C and Excel's C API and compile it
as an xll addin, which is much more complicated, but gives the best
performance and greatest degree of control over the function's
attributes when viewed in the Function Wizard.

Regards

Stephen Bullen
Microsoft MVP - Excel
www.BMSLtd.co.uk