How to calculate GMP in ipo listing

How to calculate GMP?

one famous method almost everyone following is

interest for 7 days for 1x NII subscription = P

x* number of times NII subscribed = P* n = GMP

but my doubt is how to calculate the interest for 7 days of 1x subcription

GMP=Grey market premium. Some people trade the Stock before listing .Its thats price. It will obviously be premium only when subscription > 1x. No one will pay premium when stock is guarantee available in IPO. So no premium!

copied from , in the following case they have calculated 44 paisa as the interest for ix subscription.

Shyam Metalics & Energy IPO – Market’s Current Estimates of oversubscription:

NII = 35KCrs. = ~259.31X
RII = 27L Forms = 11.81X Applic. wise (Avg allotment of ~3.81 shares per lot)

Interest cost @7.50%p.a. for 7days = 44.01paise for 1X

Thus, for NII the costing = 259.31 X 44.01 paise = Rs.114.31 per share (= GMP)

And, for RII the costing = 114.31 X 3.81 = Rs.435/- (= Kostak)

Shyam Metalics & Energy IPO – GMP Estimated
GMP 145 +/- 1
Kostak 550 +/- 10
Subject 2 @5,500/-