I am trying to create a column that calculates the average Session Percentage. The operation is quite simple: take the session Total of each row and divide it by the overall session total. So in this case the "Session Percentage" for "main" would be: 202/ (202+0+146) = 59.7%. I really appreciate the help!
What problem do you have with it? Do you not know where to start or have you already tried and got an error message?
If I understand you right you do not want to reindex all data to add the calculation result. Instead you want to add a column which is calculated just in time. In this case, Kibanas scripted fields are for you(beware that this might affect performance!): Scripted fields
So you could go to the Kibana Admin page, select the Index pattern and add a scripted field like(untested code!):
doc['main'].value / (doc['main'].value + doc['ResetUserData'].value + doc['MediaService'].value)