As for the effective RR, I calculate a RR of about 21% between 75 and 2000 ... is that about right?
Only if your name is Warren Buffett *
Hopefully a math-wiz will chime in here to help you out, but the ERR calculation is more complex than what you're doing.
Assuming you're calculating the final balance correctly (it looked OK to me when I glanced at your source), then you want to find the rate that would have given you the same balance with simple compounding, right?
The formula to calculate the balance B after periodic contributions C for N periods at rate R is:
B = C[((1+R)^(N+1) - (1+R))/R]
You know B, C, and N -- find R.