Our funding decisions were always based on our group's established mission within our school: in short-to improve the educational opportunities for our students. In line with that mission, programs and events were established, expected costs for each item were determined, fundraisers were chosen and income was estimated, and a formal budget was written and approved by the membership. We then pretty much followed the budget. If a request for funds that was not on the budget came to us, the board decided if it fit in line with the mission and if funding was available and then presented it to the membership for approval. If we had a surplus at the end of the year(a fundraiser exceeded our expectations, for example), then we requested that the principal and teachers at each grade level meet and come up with a wish list. The board then reviewed the wish lists to determine which requests were viable and presented it to the membership.
We basically followed our mission statement, budget and bylaws.
I am on the board of a PTO in Wisconsin, where we raise quite a bit of money each year, but struggle as a board to determine how to use it. We have published guidelines that are very high level, which makes it sometimes difficult to apply in our day-to-day funds approval decisions. This leads to conflicts amongst the board as to what gets approved and how much --- and then ultimately final decisions seem somewhat arbitrary and leaves us all frustrated. Even worse, the principals that are making the requests don't know what to expect from their PTO.
I've done some searching on the web for funding guidelines for other PTOs, even checked the PTO Today File Exchange, but haven't seen anything more comprehensive.