The formula for expected production would be
240 - (10 * [down time hours])
But in a single table app you would need to enter a downtime entry for every day even it's is zero.
If you did not want to do that, then there is a slightly more complicated two table design where you would not have to enter zero values for the "good days" when there is no downtime.
In that setup, you would use excel to load up all the days for say the next 5 years. You would create a date field in Excel for those days and import that column into a new table called production days. You would delete out or not upload the weekend and holidays.
The you would set the Key field of that table to be the date field.
Then you would make a relationship to the downtime table and use the date field as the reference field to join the tables together. But it will actually work better if you let the system create a new field called Related Date and then use that field but change it to be a formula field equal to the manually entered date field.
Then make a summary field of the downtime hours and then do your calculation of expected production by day on the Production Dates table.
Post back if you get stuck anywhere.