Many people I know just do daily totals. However, what I think a knowledgeable acct. would tell you is (
http://www.rbstaxes.com/) if you are multitabling the same limit at the same time, those may be counted as the same session. However, if you start and stop or are playing two different limits, those are separate sessions.
However, those totals do come out ridiculous, and I myself use daily totals/separated by limits.