Updating a field

Updating a field

Hi
 
Could you help please -- I am sure this can be done..
 
I have a Form = Clients that has a field = HealthPlan - Yes/No
 
When the Client Purchases anything :
 
I have a Form = ClinicalDetail  ( like Invoice Detail) - I have a Field = Discount in a subform called  Rows  ( main Form is the Clinical ( like Invoice) - which holds the Client ID
 
 
I would like Clients that are on the HealthPlan - to get a 10% discount - so that this applies automatically.
 
So I would apply script to Form = ClinicalDatail on Add
Fetch Records Clients - HealthPlan - Yes
Update Records Clinical Detail - Discount - Update to 10%
 
My concerns are not to mess up the invoicing as this runs m business... I will play around in a duplicate dummy database until I get it working..
 
Thanks in advance for your help..