Phoebe,
Thank you for your thoughtful response. Do you have any more details on the contingency plan? Really, we need to know what spending would get cut. To make these decisions you need to, as economists say, "think at the margins". You need to compare marginal utility and marginal cost.
That is, you need to look at what the cost of raising an extra $1000 is (including effort of the fundraisers, ill-will, processing costs, time wasted on foundation-l arguments, etc.) and you need to look at what you could do with that extra $1000. Only then can you decide if it is worth raising the extra $1000.
You can't look at the fundraiser and the budget as a whole, you have to look at just the extra bits (the margins). That means you need to have some kind of priority order on your various budget items. It doesn't necessarily have to be very precise, splitting things into 4 or 5 categories (eg. core things that we have to find some way of funding whatever happens, very important things that it would really hurt us to do without, optional stuff that will have a really big impact, optional stuff that it would be really good to do but wouldn't be that big a deal if we had to cut it) would be a good start.