I think if you were to calculate the commitment period in terms of a budget, it would depend on how you make that calculation. That calculation is still under negotiation in the Kyoto track of the negotiations.
It is also important to note that the Kyoto Protocol provides a range of flexibilities and specific rules to parties that they can make use of in order to meet their first commitment period target. It is not the case that a post-2012 target is, by definition, continuous from a 2008 to 2012 target. It really depends on the rules under which countries have to meet a specific commitment.