The only way that you can natively perform math across many rows (such as summing up all those rows in one column and dividing them by the sum of the rows in another column) is to link all of those records to the same record in another table.
So, for example, you could create a second table called “Months”. Then, back in your first table, change that month column from a text field into a “linked record” field and link it to your new “Months” table. Now, if you go back into your new Months table, you will see that you have a brand new record called “August”, and all 4 records from the first table are linked to it. So now, in your second table, you can create a Rollup field that pulls from one of the columns in your first table with the formula
SUM(values). This will give you the sum of that column for the linked rows.
In practice, you would probably want to turn the month field into a “month & year” field, so it doesn’t get messed up next year with records showing up in August again.