Calculating Averages problem for use in Reports

Calculating Averages problem for use in Reports

I am hoping that someone can help me with a way to accurately display information I am capturing in Creator into Reports.  The just of my form is that the user fills in the number of safe and unsafe observations of a given activity.  The sample data would be as follows:
1.  3 safe, 2 unsafe
2.  90 safe, 10 unsafe
3.  1 safe, 1 unsafe

I have built in formulas behind the scenes of the form to calculate the total number of observations and then also the % unsafe of each record.  So for these records it would be 40%, 10%, and 50%.  In reports I want to be able to track over time the % of unsafe behaviors occurring.  Thus my x-axis is full date.  For the y-axis, I pull the %, however I have no good choice in terms of what to do with these values.  I obviously don't want a max, min, sum, etc.  I need to do a weighted average.  Assuming all three of these records were for the same date, the correct weighted percentage the report needs to display is 12% (13/107).  However Reports will just average the 40/10/50 and return 33%.

Can anyone suggest how to accomplish this?  I'm not sure whether or not the changes need to be on the Creator or Reports side.