Having filed my I-864 last year, 2024, and not having made any additional income at the time of filing, I just used what was reported on my last paystub cumulative-annum. I did not update it for 2025 and the CO did not seem to mind or care.
Since it is an estimate, and I have never heard of them asking you to verify _or_ circling back to it if it seems fishy, nor have I seen explicit instructions from a verified source on defining how the calculation is made, AND them seeming
to really only look at recent paystubs/tax transcripts, any number that looks about right would be okay IN MY HUMBLE OPINION.
If you took two weeks unpaid for a medical visit or something, for example, I'd be hard-pressed to be convinced that my annual income should be $0 * 26 weeks.
Just make sure the beneficiary is able to back up whatever number is listed with currently submitted documents (paystubs, tax transcripts, letter of explanation, etc)