BigInch's approach will probably get around the issue of "droop" vs "continuous rise to shut-off" that I have often seen in pumps with flat characteristics. I don't know if that is the primary intent though. I have also had a bit of success adding a fourth term c4*[e^Q] to flatten out the curve left of BEP a bit more.
The advantage to using an equation is that it makes (in my mind) the Excel logic easier towards manipulating curves using affinity laws and determining intersections with system head curves.
Surprisingly, the best curve fitting program I have ever used is an old DOS Shareware program called XYMATH, although I use CurveExpert32 now and that seems OK for most of my needs. I haven't played with Excel's built-in curve fitting much, but based on what I've seen to date, I prefer the two mentioned above. Both are "free" off the internet.
Regards,
SNORGY.