It's not the formula that is hurting my head at the moment, it is determining which elements of information are to be used in said formula. I don't even use the formula I just use the CR3 much quicker and has been treating me well up until this moment. I had it all figured out and then reading a more accurate comment somewhere else that true altitude is really applicable to a column of air above a point on a sphere. Which stands to reason that AGL is the number that should be used and not AMSL. Which would make this question redundant because he's asking for a height above a specific point but is not providing an accurate QNH source.
Thanks xCartzy, but it doesn't change the answer by enough to justify that Higgins still says that the answer is 188ft, which you only get when using AMSL figures. This is what is baking my noodle; even if you allow for pressure height, the answers you get when using AMSL or AGL are different by hundreds of feet
When working off pressure altitude, in AGL;
4ft x (940/1000) x 14 = 56.4,
7000 - 56.4 = 6944
6944 - 6420 = 524.
I know the CR3 isn't the most accurate machine, but it's not out by several hundred ft unless you've done something wrong. I'm just going to assume that the questions in the exam are going to be asking for a height AGL and providing an accurate QNH, not area QNH. This isn't my first ATPL rodeo, and I know what CASA is trying to bleed out of you, I just wanted to check that I haven't spent hours building a working knowledge on a concept from an incorrect source. Which I'm going to need tomorrow haha.