Originally Posted by ArviLaanemets

My advice is to use table like one in sheet Invoices. From your table it looked like payment terms are fixed for client, so I added a sheet with Clients table where payment terms are stored.

In case payment terms can vary for same client, you need a column for them in Invoices table too (and remove payment terms column from Clients table - and edit the formula in payment terms column accordingly).

The main difference between 2 designs is, that with your design, you have to add new columns into your table whenever you register new payments. With my design, you simply add new payment entries into table. Also, whenever you will have new clients, you simply register it in Clients table, and it will be all you need to add invoices for this new client.

You can also have any number of report sheets designed, which read data from Invoices and clients tables, can do any calculations based on read data, and display the results in any format you want.