If the Lead time in days has been maintained as 20 days in your document--then why are you applying the same sales document to a customer-who requires less than this 20 days???
Why dont you take one more sales document with 0-Lead time in days(Possibly)--can be applied this document type to the customers who require goods faster....
Let me again share with you ---Actually system finds first the material avbl date at plant level-then only adds pick/pack/load time...
Phanikumar