Hello @Tori2,
correct currently Asana only allows you to sum fields but not multiple.
I suggest to upvote here: Computed Fields on Tasks - #32 by Bastien_Siebman
Good news is that there is a solution for you 
Check out this message from Bastien: Computed Fields on Tasks - #34 by Bastien_Siebman
Jerod needed the same feature which is a probability field and he found a solution. Check out his post.
Hope that helps 