VAT - old scheme D or Direct calculation scheme?

Hi, It's been a while since I have set up a Direct Calculation scheme (or old VAT scheme D for those old enough) for a client, as they all seem to be on EPOS these days.

But this is a new client who hasn't got one, small rural petrol station with CTN shop, so I have recommended scheme D untill they can afford a new EPOS system. I know I have to seperate out the zero & lower rate purchases, and then calculate them up to their Expected selling price, take off gross sales etc.

My question is - is there a quick way of deciding margins or are there any set margins anyone knows of?

Or do we have to trawel through their supplier invoices looking for RRP, for first quarter and then average?  

Any help appreciated to reduce time & fees, as they intend doing it themselves once set up.

Thanks