Is this figure to be before or after deducting sequestrations?
Good question. If you mean manmade sequestration I guess in this context it's preferable to stick with what we know we can do - and set aside unproven or speculative (insufficiently researched) techniques. Given that proviso it would seem reasonable to define the figure as after deducting manmade sequestration as we're talking about our net effect. It would need to be sequestration that could continue for as long as the emissions though?
If you mean natural sequestration I'm not sure we should presume upon that (at least not over short term timescales) - firstly we don't have unlimited capacity here, and secondly some natural sinks for carbon dioxide are still problematic (for instance ocean acidification) or might even definitively become net sources in the not too distant future.
[EDIT] Just to be clear, I don't think we need to put a lot of thought into this particular value - just to find by consensus a reasonable ball park value in the context of our individual understanding - precisely the same activity that would need to be done at some point by the wider public or international agreement.