how to create a field percent formula with a sum value.

You will need to create a new table to accumulate the totals.  For example load up a table of dates with the first day of each month and make that the Key field of the table.  Then make a formula field to calculate the first of the month and use that field to connect to the summary table. Then make your your totals in the relationship and do the calculations there.

But if you have complex breakdowns by product, for example, then there is not an easy way to do that.

It is a complex breakdown of a large number of different parts. I have individual scrap % as this is easy.

There is no automatic easy way just to divide those two numbers.

In a single user environment You could set up a single record in a single table where the record id is 1.

Then you could relate that table to every record in your details table through a relationship where the reference field is a 1 as well.

Then you would have any filters on that single record such is some date boundaries and then summarize your columns on that report up to that record ID number one. And you would be able to do your calculation there. That would give you some flexibility in the filters on the report and give you your totals.

This could also be extended to a multi use your environment just so people are tripping over each other when they are editing that filter record ID number one