customize the Profit margin % calculation in sales order

Hi SAP Guru's,

Our requirement is to customize the Profit margin % calculation as like below:

GP% =( Net Sales – VPRS)/Net Sales

Where Net sales should be =
{Base price – (Rebate condition amount values)} – Discount (as per SAP, Net price= Base price – Discount)

So to achieve this we will go for VOFM routine only or we have any option to achieve this through user-exit.

(because in user exit am getting values only vprs and not Profit margin % value)

If yes, please let me knowthe exit, I tried with MV45AFZZ Exits but here we are getting only VPRS value (1010.00)
please find below screen shot for reference.

So any one help on this please.

Thanks in advance.

Regards,

Shabareesh