Indeed, usually, the Current Year needs to have some kind of Forecast process in order to have a Full Year of the current month... and this is driven by the Forecast Process of that particular company :
Generally, a Forecast process starts with an initialization.
For example, for a Forecast 9+3 ( (9 - Actual months, 3 - Forecast Months)
- Actual months: import actuals from legacy systems (or any other source where are the actuals)
- Forecast: Copy data from the previous Forecast cycle.
Usually, this is the starting point of the Forecast cycle and after this, the users tune this initialization in order to have the most updated data for the Forecast months.
So, in your case, if until September there are no other Forecast cycles, the best option I would choose: consider for the last 3 months of the year the Current Year Budget.
If in the company there are previous Forecast processes ( for example 6+6 Forecast), consider the last 3 months, the previous forecast cycle.
Answer to this question: what is the best-updated option in order to have the most updated value for the current year Forecast months.