Please Help- Scheme of Vat (Mumbai)

my client is having food grain business, with other 4% and 12.5 % items

which scheme should be best as 75% items are foodgrains which are tax free. ?

should i go for set off scheme, as the tax burden is less then the composit scheme. ?

what form of additional accounting to be kept in set off scheme. ?

if he go in composit scheme, can he calculate GP only on taxable items ?

please help