I believe you can't just retrieve these values from your table chart object, e.g. by creating a variable within your chart.
(Well, I am not absolutely sure if you can achieve this with a macro, but I haven't seen this so far. And using macros for this seems a bit heavy, i.e. you also need to take care of triggering the macro if the value changes).
You can try rebuilding these total values in your variable definition in variable overview, though. In a simple setting, the total will be just the expression total in global context, so just copy your chart expression as variable definiton. If you use some kind of restrictions in either dimension (calculated dimension) or expression (e.g. set analysis), this could get a bit a immense more complicated.
If you could post a sample file, maybe someone is able to help you with the variable definition.