Schedule Arrival Date calculation

Hi,

We have MAKE TO ORDER (MTO) items with 10 days lead time and MAKE TO STOCK items with 1 day lead time. And it takes 6 days to ship items to the customer site (In-transit days = 6).

1. If order date type is 'Ship', Schedule ship date for MTO item is today+10 days and Schedule arrival date is today+ 16 days (item lead time + in-transit days) - This scenario works perfectly.
2. If order date type is Arrival, Schedule ship date for MTO item is today+ 4 days and Schedule arrival date is today+ 10 days. We need at least 10 days to make the item but the system here calculates SAD first with 10 days lead time and then derives SSD. We cannot make our MTO item in 4 days as system suggested. Our expectation is that the SSD should remain today+10 days irrespective of order date type value. We have this issue mainly with Internal sales order, where we cannot change the order date type. Can you please shed some