I'm trying to use this:
SLOPE( INDIRECT("G"&ROW()&":G"&(ROW()+I2)), INDIRECT("O"&ROW()&":O"& (ROW()+I2) ) )
where the contents of I2 is a positive integer.
It doesn't seem to work. Getting #NUM!
And, very hard to debug because SLOPE requires two ranges.

It must be either something really dumb or SLOPE doesn't like INDIRECT??

What I'm trying to do is evaluate relative slopes computed over different lengths. So, I2 contains the current length.