Weekly Amount Sum + X

Im not very good with formulas so I am looking for help.

I log most of my business activity in an Airtable database so that at the end of the month I can invoice my customers. I found a formula here so that I can calculate and view weekly dollar value sales totals and it works great however there is a known quantity of work that I don’t log and I want to add that known amount to the calculated value displayed.

To get the weekly sales value I have a Formula column with this calculation - DATETIME_FORMAT(Date,‘w’) and I have a view where I group by this column and it sums the amount column in my database.