Hi Nikhil,
Create different pack spec for each type of quantity classification.
PS1 for box
PS2 for pallet.
Define two different logical conditions, one for quantity <= 20 and another one for quantity between 21 to 50.
LC1 qty <= 20
LC2 qty btw 21 to 50.
Define condition records and with above logical conditions to determine the correct pack spec.
Regards
Srinivas.