This got 12.44% for year 2. [....] Any clue as to why their [....] returns leveled off at around 14% in years 10-30.
There are many ways to measure a "rate of return".
Your formula in column J is
not how to calculate the IRR per se.
Instead, you are calculating the
simple rate of return over n years, n=1, 2, 3, etc. And even then, I believe that you are calculating that incorrectly. See below.
The values in column K can be calculated using the Excel IRR function.
The IRR is the discount rate that sets the NPV to zero. See the Excel NPV support page for the mathematical formula.
-----
First, I believe the formula in column I (I call it "net value") should be:
I4:
=F4+H4
Copy I4 down the column
That is, it is the accumulated equity (H4) plus the net cash flow (F4), which is assumed to be added at the end of the year.
(I am taking the values in column H for granted. It is unclear if and how they are calculated. Notably, it is unclear if and how they relate to values in column G (COCR).
-----
Then formula in column K is:
K4:
=IRR((F3,I4))
K5:
=IRR(($F$3:F4,I5))
Copy K5 down the column, formatted as Percentage.
That is, the IRR cash flows are: the initial outlay (F3), followed by n-2 net cash flows(0 for row 4; 1 for row 5; 2 for row 6; etc), followed by the net value (I4).
Note the syntax of the Excel IRR formulas; that is, the parentheses around the list of ranges. The Excel IRR function is one of very functions that permits that form of a "range union".
-----
I believe the formula in column J should be:
J4:
=I4/(-$F$3) - 1
Copy J4 down the column, formatted as Percentage
That is mathematically equivalent to -I4/$F$3 - 1 , if you prefer.
The results are -2.43% (-2.43109559613319%) in J4, 2.90% (2.90298961689939%) in J5, 8.43% (0.0842583243823845) in J6, etc.
-----
If you have further questions, please post all data and formulas, ideally in a table generated by the new XL2BB application.
Alternatively, upload an example Excel file (redacted) that demonstrates the problems to a file-sharing website, and post the download URL in a response here. I like box.net/files. Caveat: Some contributors object because they cannot or will not download Excel files.