**Extra margins with assume thrust**

Hello you all! I am flying the 737NG:

I was told that if you use assume thrust, you will have the additional margin for take off between the density of the assume temp and the colder OAT. If i understand it correctly, if you have flat rated engines at 30 degrees C, the engines will adjust N1 and EGT to give a flat rate whenever it is colder than 30 degrees outside. Therefore you cannot select a colder assume temp than 30. But in my mind then, and here is the real question; you will only have the density margin from the assume temp down to 30 degrees then, and not all the way down to lets say 4 degrees C?

I hope this makes sense, here the mathemathical way of asking the same:

With assume thrust of 40 degrees, and OAT 4 degrees, flat rated engines at 30 degrees, Is the density margin then:

A: 40-30=10 degrees colder with the better density that gives, or

B: 40-4=36 degrees colder with the better density that gives?