Well since no one answered, I supposed I needed to find some other way.
As my values are similar to the y of a parabolic curve, I found the coefficient of a 3rd degree polynom, which approximates this curve.
This is made by filling a formula in an array of cells 1x4:
worksheet.MyRange1x4.formulaarray = "=LINEST(" & MyX & "," & MyY & "^{1,2,3})"
Where MyX and MyY are again arrays with the same dimention (1 x n)
The results are the "a", "b", "c" and "d" from the polynom ax^3+bx^2+cx+d
Now, if I wanted to find the X of my maximum I just had to solve a delta, which is very similar to the one of the second degree equations: Delta = (-b (+/-) SQRT(b^2 - 3ac))/3a
Forcing the Delta to be 0, I obtained 2 possible X values (because of the +/-SQRT), of which one was definitely in the range where all my nearly maximum values were, and the other was totally wrong with this range.
Selecting the correct one made me finally find my X out.