Standard Packs - balancing cartons with > 1 std pack but < 2 std packs

Has anyone devised a method to "balance" the quanitities in Standard Packs when the order quantity is greater than (1) standard pack quantity but less than (2) standard pack quantities?

Example:

Item "X" standard Pack is 20

Order quantity is 30

Standard Pack will assign 20 to the standard pack carton #1, then carton #2 will contain the remaining 10 of the order quantity of 30.   

My attempts to "reorg" the carton #1 to "balance" the quantity in each carton - by removing 5 from carton #1 to then "add" to carton #2 - results in an error - Line number Invalid".  In fact the line number is the EXACT line number detailed for carton #1.  NOTE: I can "add" to carton #2, but the issue is to "remove" from carton #1.