[gizmo686]

Excel supports iterative calculations, which allows you to use circular references. If you enable it, you need to specify a maximum number of iterations excel will run on each update. I believe you can also specify a minimum delta which will cause Excel to terminate early if subsequent iterations differ by less than the delta. I suspect the reason you need to press f9 repeatedly is this project uses circular references, but running your program takes longer than the maximum number of iterations, so pushing f9 allows it to run for another set of iterations.

[Jtsummers]

That makes sense, I've never made use of that. Apparently that was available as early as Excel 2007, I've just never come across it and haven't used Excel for much more than a scratchpad in years now.