Hi,
"Should allow max capacity 30Gram and beyond it any quantity should go to reserve area."
OK, you can control the max number in a storage type (NOT in a bin) with the flag "Check Max.St.TypeQty" and maintaining the max quantity in the warehouse product on storage type level (what Petr described above). But you need to split in order that this happens the way you want that.
But you are aware that this whole thing is not really very clever (and I do not mean the unit of measure)? OK, but understanding how the cap check works is of course important.
Brgds
Juergen