@jayumiya
To calculate interest, first you need "approx" figure of NII subscription. As provided by Eagleye mam, it would be 666.67X in the above example.
So, if as a NII, you apply for 666.67 lots in your application, you would get confirmed 1 lot. Now, let's calculate how much money you would need to apply 666.67 lots.
1 lot = 14,400 INR
666.67 lots = 96,00,048 INR
So, NII would need to borrow 96,00,048 INR to apply for 1 lot. Let's assume that they borrow this amount of money @6.50 interest rate. And as you know, generally it would take 7 days for the allotment, and after that your money would get released. So, NIIs have to borrow the money for at least 7 days.
Now, let's calculate what is the interest amount for 7 days, if one borrow 96,00,048 INR @6.50 interest rate.
After calculation, the interest amount comes around: 11,967 INR
So basically, to get 1 confirmed lot (50 shares in case of this IPO), NII need to pay 11,967 INR interest. So per share, you're paying around 240 INR (11967/50) interest amount. And that's what indicates the GMP.
So if the IPO doesn't list at ISSUE PRICE + INTEREST COST OF A SHRE (288 + 240), NII would be in loss, since they won't be able to cover even the interest cost (forget about profit).
Also, if you divide 240 (interest cost per share) by estimated total subscription (666.67), you would get interest cost in "paise" for 1X.
240/666.67 = 0.35 paise for 1X
I might be completelly wrong, but that is what I understands about how the interest is calculated.