Entering GL distribution defaults by amount is not offered when adding new Products and Billing items because the total amount on the Billing Item (Item Tab) is not required.  For example, let’s say you wanted a Billing Item that is always split between 10 accounts by a specific percent yet the amount of the item fluctuates.  If we allow distribution by amounts, we would have to check the amounts entered on the GL Distribution tab against the total amount on the Item tab, and you would never be able to leave this field blank.  Because we have to support the ability to leave the total on the Billing Item blank, we decided to only allow distribution by percentage.