Synthetics + Transforms

Hi Team,
I have a total of 80 monitors in synthetics with polling interval set as 5 minutes. This data is saved in index synthetics*. However, on top of it, I am using a transform to fetch the latest data from this index with the retention of 7 minutes policy. On the discover what time should I set to get the exact count? I have seen setting the time as 7 miutes to 11 minutes, but the number of documents keep on changing. What logic should I apply to get the right count of documents?

//Ankita