How to calculate Percent of total

I am trying to create a cohort analysis with the % retention, month after month.

I have created a cohort table with absolute numbers, but I cannot calculate a percentage of the total for each cohort.

For example: I am trying to calculate the numbers within the pivot table as a percentOfTotal, where the total should be the distinctCount of created_at for each month.

Does anyone have any ideas ?

Thanks!

Hello,
Could you please share a small sample of “fake” data replicating your dataset for this table (10-12 rows).
If you can also include an screenshot to understand on each cell what is the meaning of each value? E.g. crated_at=May/Send_date=Jul2021, what is the total at that point?