As I remember from school days,the braking force =M*g*m(braking coeficient)
As you see it's directly proportional with the mass of the object.
This is the reason for the spoilers after landing,to spoil lift-resulting mass increase-resulting higher braking force.
At 48 T (i.e.) the braking force will be lower than at 53T ,so the reason for lower V1(48T) than V1(53T)
So,if you use V1(53T) for 48T -a risk to not be able to stop.
This is why i'm concerned about this assumed procedure( increase V1 for lower weights)
Of course,somewhere I may be wrong,I just don't figure out where.