Under a Cat F application you can use gross taxable income from the last full financial year.

Under a Cat G application you can indeed utilise the average gross taxable income received in the last two full financial
years.
So for example if you had a gross taxable income of £16,800 in the last full financial year, but in the previous financial
year you received an income of £20,400 the average income from the two financial years would be considered as £18,600

The specific details and mandatory supporting documentation can be found in Annex FM Section FM 1.7: Financial Requirement
Please do take time to review. UKVI are very strict on this