need help with NewRecord expression

[Slez via AccessMonster.com]

I am creating a form where users enter new records that relate to the field
[JobNumber]. I am wondering how to write an expression or code that will add
1 to the New Record in the field [PunchID]. It would be sort of like an
AutoNumber, but each JobNumber would start with "001". Here is an example:

JobNumber PunchID
065555 001
065555 002
065555 003
- new record would default to 004
065577 001
065577 002
- new record would default to 003
...and so on...

JobNumber is the one side of a one-to-many relationship with PunchID.
Thank you in advance for any help!

 

[Klatuu]

NewPunch = Nz(DMax("[PunchID]", "MyTableName", "[JobNumber] = '" &
Me.txtJobNumber & "'"),0) + 1