Quotation to Sales order with QTY block

Hi,

I want, while creating Sales order from the quatation if the Qty in sales order is less than 25% of Qty in quatation. Order should not be able to process (Block) & it will ask for creating another quotation.

does it requrie baddi or user exit & if yes which user exit should i use,

Pls reply.

Thanks in adavance

chetan