What is the process for reducing line/schedule/multiple distributions quantity/price in BWC

Trying to reduce quantity/price (depending on line type usage) appears to be different in BWC than it was in forms.

Forms process was to update quantity/price to match qty billed/amount billed.  Upon SAVE shipments (schedules)and distributions was updated.  But in the case where you have multiple distributions, strange things start to happen, especially if the multiple distribution needs to reduce to zero.

1)  What is the best/trained process to reduce to the quantity/billed value with mutlple distributions?.

2)  BWC update of line quantity/price does update properly to schedules.  There appears to be a calculation being done, why?