yes, if Cm slope is negative, "real" AC is aft of the 0.25 c (or to be precise, aft from the point for which Cm graph is given). That stands both for "normal" airfoils, and reflexed (Cm>0). This is not contrary to graph in post 24. Everything is fine there.

I think you should try to skip calculating "exact" AC position, as this data is pretty much meaningless. XFLR Cm chart in respect to known-fixed point (25% chord) is all you need here and that is accurate - I mean the approach - program as such has its errors and limits of course.

there is one more thing regarding "real" AC position. Straight Cm line, whether tilted or horizontal, means there is AC in some fixed point. That is the only case there is sense to find AC position, though I dont see it as a practical approach either. However, Cm plot is straight just in theory (like idealized in picture shown), in reality it is often more or less curved, which means that AC moves with AoA. So, AC in most real airfoils is not a fixed point, but it does travel (not much but still does). Whenever you see bumpy - curvy Cm vs AoA chart keep in mind that that airfoil doesnt have fixed AC.

You can check that yourself. Take ronczs spreadsheet and try calculating AC for different spots: in cells D4 and D5 insert Cl and CM for other Alphas... 2, 5, 6, 10... and so on. If you insert CL and CM for AoA=4 (that is by default, but doesnt have to be that) you will just calculate averaged AC over domain from CL=0 - CL@AoA=4.