Custom recognition events - Custom Percentage

Hello All

we are currently in discovery with NetSuite regarding our implementation of NetSuite but struggling to get a convincing answer on one specific need we have. I am not a finance person, but am involved in the systems integration aspects. The fundamental need we have is as follows:

There is a standard Recog plan for Project percent complete. This applies the percentage complete to each line of the Sales Orders based on other info like dates. This is the basic plan we need, but with one change. Rather than using the percentage complete from the project, we want to have a custom field on each sales order to represent the percentage complete for the lines of that sales order. This field should drive the recognition plan. I keep getting told we cant do it, but find it hard to believe that this cannot be customised?