It depends a little on how this db is going to be used.
The easiest solution is to have a relationship between Projects and Quote based on your primary key (you should have that relationship regardless). Then in the Projects table, define an unstored calculation, say Quote Count = Count ( Quote::_kp_QuoteID ). The in the Quotes table, your user-visible Quote ID field = Project::Project ID & "." & ( Quote Count + 1) .
If this db is heavily used by multiple users, you may run into trouble if two people try to create a quote for the same Project at the same time. If that seems likely, there are other ways, such as having a script control the creation of new Quotes and a table that tracks each Quote ID. But that's probably more work than you need to do.