What you calculated was the distribution yield - the interest paid at the end of a given month or quarter, divided by the NAV at the end of the month or quarter.
SEC yield takes a look at the securities held by the fund and determines the future rate expected to be paid by the fund. It’s a bit more obscure. It. Is forward looking and annualized. You can expect a distribution yield to gradually drift up or down to the SEC yield. SEC yield is actually a good estimate of a bond funds future return assuming stable interest rates.
I often see a wide divergence between what a fund pays out in a given month and the SEC yield. I think the SEC yield is useful to compare funds and get a feel for future performance. But it’s not going to tell you what percent you’ll be paid today. If the SEC yield is lower than the distribution yield, you can expect interest/dividend payments to drop, and vice versa.
A bit more info: https://www.thebalance.com/distribut...you-use-416978