Your question appears to assume that maximizing the net present financial value of your donations is optimal, but you should really be concerning yourself with the effectiveness of your donations, rather than their size. There are plenty of causes time sensitive in nature (climate change being just one, stopping spreads of diseases, etc), and its one of the reasons the Gates foundation plans to spend itself out of existence rather than existing in perpetuity. As velocistar237 said, you need to pick your metric, but what causes are important to you, and when in time your money can be spent most effectively by the organizations working on them should factor in heavily.