I believe the developing world will settle for the 80/20 split on who pays - the developed world paying the lions share.
The US is unlikely to play ball.
The following paper provides an analytical approach
Equity in Climate Change: An Analytical Review [PDF]
How global emissions reduction targets can be achieved equitably is a key issue in climate change discussions. This paper presents an analytical framework to encompass contributions to the literature on equity in climate change, and highlights the consequences—in terms of future emissions allocations—of different
approaches to equity. Progressive cuts relative to historic levels—for example, 80 percent by industrial countries and 20 percent by developing countries—in effect accord primacy to adjustment costs and favor large current emitters such as the United States, Canada, Australia, oil exporters, and China. In contrast, principles of equal per capita emissions, historic responsibility, and ability to pay
favor some large and poor developing countries such as India, Indonesia, and the Philippines, but hurt industrial countries as well as many other developing countries. The principle of preserving future development opportunities has
the appeal that it does not constrain developing countries in the future by a problem that they did not largely cause in the past, but it shifts the burden of meeting climate change goals entirely to industrial countries. Given the
strong conflicts of interest in defining equity in emission allocations, it may be desirable to shift the emphasis of international cooperation toward generating a low-carbon technology revolution. Equity considerations would then play a role not in allocating a shrinking emissions pie but in informing the relative contributions of countries to generating such a pie-enlarging revolution.