- Roughly 1/4th of the SUM'd yellow column is the earned amoritized interest

I was following until you did the "1/4". Why 1/4?

They don't make this easy, do they? It is now nearly impossible to get at the breakdown of payments (p vs i) on the web site.

Here's a different approach. Go to your monthly statements. Look at the "earnings summary". Look at the "This period" column. Write down the "total" from each month. That's what you earned that month. All the interest, writeoffs & fees, and none of the principal repayments. You can also look at the "account total" box and write down the total account value at the beginning of the month and divide. If the total earnings is $1 and your total account value was $100, then you made $1/$100 = 1% that month. 1%/month annualized is 12.7%/year.

This approach neatly removes any need to subtract or adjust for principal payments or payoffs.