I dont know how to write the expression to get it to sum for only the Maximum forecast Submission which is stored as ForecastDate. I dont mind if I have to use a variable to get it to work. I have tried that as well but cant get the Set Analysis to work there either.
You're expression looks correct. If it doesn't work that could be because you misspelled a field name, Qlikview is case-sensitive wrt field names. Or ForecastDate contains strings instead of numeric dates causing max to fail to get the max date. If you need more help post a sample document that demonstrates the issue.