"A Simple Offset at Châtelaillon-Plage #2" Multi-Cache

You are at Châtelaillon-Plage. A little village where they live from the sea. Most people here do proffessional fishing the Mussels and Oysters. At this place there are some locations to hide a cache and have fun with the geocaching stash hunt. This one is a simpe offset cache. that's means that you have to solve puzzles at the start location, to get the coordinate of the stash location. Okay, lets start:

The endcoordinate is at :

N 46 03.ABC
W 001 05.DEF

At this coordinate youre standing on the way, look down under your feets, there is a manhole cover made from steel and have a great weight. 
At left upperside (or right bottomside upsided down) you see a little round with the characters NF on it.

Question
What's the three digit number at right side of NF.

The answer is XYZ

To calculate the stash coordinate calculate the value for each character and fill out the stashcoordinate.

A = X
B = X
C = Y - ( X + X)
D = C
E = D 
F = Y + Z