Best incremental Invoice formula

Looking for the best Invoice incremental script. I have been using the Trigger on Create 

let c:=cnt(select 'Invoices');

'Invoice Number':= "INV-" + format (c,"0000");

New Script;

New Script; ect ect

I thought this was working well until I realised I am getting repeated invoice numbers. Is there a best practice and where should the script go? I have seen another script that 

let before:=max((select Invoices).Invoice Number);

Invoices:= before + 1

Why do you use this script over the other, can you add extra scripts after it and where do you put the script?

Thoughts and suggestions appreciated 

Michael