That's why Effective Altruism movement settled on the 10% of income as a default "good enough" contribution. As Scott Alexander explained in [0],
"It’s ten percent because that is the standard decreed by Giving What We Can and the effective altruist community. Why should we believe their standard? I think we should believe it because if we reject it in favor of “No, you are a bad person unless you give all of it,” then everyone will just sit around feeling very guilty and doing nothing. But if we very clearly say “You have discharged your moral duty if you give ten percent or more,” then many people will give ten percent or more. The most important thing is having a Schelling point, and ten percent is nice, round, divinely ordained, and – crucially – the Schelling point upon which we have already settled. It is an active Schelling point."
It's still better to get a suboptimal outcome than to not get any outcome at all.
[0] - http://slatestarcodex.com/2014/12/19/nobody-is-perfect-every...