If user wants to generate rate on the bases of margin so then user needs to follow below steps:-

1. Go to Master > MargSetup > Control Room > Search in all > Search -- "Fix" > Click on Accept > There is one option shown > "FIX CALCULATION ON MRP for Sales & Purc.Rate Y-Item F-Fix N-Not Req." ---Change that option as "Y" > Then press esc key and save. 

2. After changing the option from the control room a Sale Margin & Purchase margin field will be open in item detail.

3. Go to Master > Inventory Master > Item Master > Select Item > Press F3 on item > In next window use will mention the Sale & Purchase margin according to the requirement > Then press Ctrl+W for save.

4. After following the above steps when user will feed MRP on purchase window then purchase and sale rate auto generate according to the Margin i.e. which are define in item master.