hello guys i want to ask, in sale.order we have sale.order.line with qty_delivered, after i deliver the product the rounding is right it's always going up, but when i return the stock picking later in my total qty_delivered sometimes the rounding is going up and sometimes is going down,

like example: 

in sale.order i have a quotation for 1 cardboard of a product, in that cardboard, i have 24 bottles, but the quantity picked only 16 bottles so it will 16/24 = 0.6666 -> it will be rounded 0.667 (this is the right rounding) but later if i do return for 2 bottles the rounding is wrong, 16-2/24 = 0.583333 -> it should rounded to 0.584 but it always 0.583.. 

but if the quantity picked is 17 (it means 17/24) then i do return 3 of them it will make 14/24 then the result is what i expected, it's 0.584, why rounding isnt consistent?

please help me why it happened and how to make it consistent to always round "UP", thanks for the answer