I did some testing and unfortunately, the functionality is that Time Spent On Site records two metrics: the Time Spent On Site and the Time Focused On Site (2358|2314). There isn't a way to alter this.
What i want to have is a split of "Time Sent On Site" and another one with "Time Focused On Site". In this way i'm able to use it in the dashboard. The ultimate goal is that i create buckets like this 1-2min], 2-5min],...
=IF((@@Time_Spent_onsite2]]>600;">10min"; IF((@@Time_Spent_onsite2]]>299;""5-10min]"; IF((@@Time_Spent_onsite2]]>119;""2-5min]"; IF((@@Time_Spent_onsite2]]>0;""1-2min]";""null]"))))