If your loose quantity doesn't contain any bag, then your quantity will be like that
20 Bag = 510 Kg
In that case, at first you have to change the conversion from MSC2N as
1 Bag = 25.5 KG. Then when you will go for issue you can use 21 Bags, system will propose the quantity 20 X 25.5 = 510. (But the problem is that, some times it becomes some decimals)
Or if your loose quantity contain a bag, then you can issue 20 bag first which has the 25 kg per bag. Then go to MSC2N and change the conversion as 1 Bag = 10 Kg, then you can issue the 1 Bag and 10 Kg.