Rob, I think the problem here is that you don't know how to use KSPread.
Here's the correct syntax for your examples (untestd, but should work):
=F7 * ( 1 + MAX( 0.20 ; (F7-F2)/F2/5 ) )
In fact, the one that you mention as working, MAX (A1:A2) works by coincidence (Unless you really want the max over a range, instead of the max over two cells).
Can we get that not-grumpy week anyway?