Its 28000. For 'manufacturers' its 1% of the total turnover including exempt,export,non taxable Turnover. So 1% of Rs.28lakh is to be taken. Only incase registered person was 'retailer etc' then you can exclude non-taxable turnover.
I thought including the non taxable turnover was to only identify if the person is eligible for composition scheme. But for calculating the tax I have calculated gst on the taxable amount only.