WF for Accounting Document line entry

[Govindarajan R, Consultant/PWC]

We are in 4.6B.
I have developed workflow for sending mails when a sales order is
changed.
Now I am trying to develop a workflow for sending mail when an
accounting document (Vendor Payment or Customer Receipt) beyond a
certain value is created.
Accounting value is stored in BSEG table and Header table is BKPF.
The steps that I am planning are :
1. Create a subtype for BSEG say ZBSEG.
2. Create an event for ZBSEG say ZCREATE.
3. Create this Event ZCREATE with condition BSEG Document type = X and
BSEG Value = Z.
4. Build desired attributes in the subtype.
5. Build a workflow based on this newly created Business Object.
 
I am stuck at step 3.
Can somebody help me in
a. is this approach right?
b. should i work in bseg or bkpf?
c. how do i create the event in the above approach.
thanks in anticipation
rgds
govind