Complex calculations using the stats function in Report Templates

[TL2]

I would like to calculate some stats in my report bason the stats function:

${mainLayer | where:"status='Pass'" | stats:"count,objectId"} / (${mainLayer | where:"status='Pass'" | stats:"count,objectId"} + ${mainLayer | where:"status='Fail'" | stats:"count,objectId"})

I am not sure where to add the operator or if you can only calculate fields and not stats.