add this line to your main beancount file:
2000-01-01 custom "fava-extension" "monthly_expenses" "{'accounts':['Expenses','Expenses:Life','Expenses:Life:Transportation'],'account_used_to_get_valid_month':'Expenses:Life:General'}"
accounts is the list of the accounts you hope to check in a deeper level.
account_used_to_get_valid_month is the account used for calculate number of month should be averaged (for now, months with more than 10 transactions considered valid).
Inspred by beancount-mutiperiod-reports. Thanks.
I like the idea of Envelope Budgeting a lot. And I also found fava-envelope, which I tried but failed to get working.
I ended up realizing that having the averaged expenses is what I need. I use them to compare my spending with my implicit budget.
So I wrote this, as a simplifed way to implement envelope budget.