Our situation is a little different as we have a “quasi-endowment” donated from founders that functions as a reserve fund but:
In 2023 I pushed for us to form an official finance and investment committee of the board and I would highly recommend that if your board has capacity. Committee size is required to be 3-5 members, including the treasurer. The board votes to approve any drawdowns after committee review and a recommendation to the board. The proposals for drawdown have come from staff based on annual budgeting needs, but have been reviewed by the committee, recommended, and then approved by the board. The committee also meets with our investment manager before each board meeting.
We require projects to have three months cash on hand but recently started recommending six months if they can manage it, so I would probably mirror that and go 6 months now for a reserve fund…especially for an environmental group in this funding environment (sorry :/).