Not having seen the Budget Reports functionality, I'm not sure if something like this exists. But given the lack of much help on the "Create Budget" screen, I'm guessing not.
As a user, I would like to be able to create a budget based on past numbers. E.g. something like this:
Create budget screen:
1. Set a budget time frame - monthly, quarterly, yearly, and date start.
2. Choose base date range to base budget on - select date start and accrual/cash method, or select an existing/previous budget
3. Hit go, and get a budget entry screen populated with actual numbers from the base period.
User should then have all accounts with activity in that screen, and be able to edit the numbers as desired and then save the budget.
In budget reporting, it should be quick to retrieve the previously saved budget for the month, quarter, or year, and compare side-by-side with the actual numbers for those periods.
I would really like to see a broader report, like a spreadsheet, with multiple budget periods -- e.g. "Show me my budget for the year" and have it show columns per month, subtotals per quarter, and totals per year. While blue-sky thinking here, I'm picturing each account in a row, and columns for the budgets, probably 3 per item: budget amount, accrued amount, cash amount.