opengl – Help to find coordinates for the left of the screen in this frustum

I have a frustum with left = -2, right = 2, close = 0.5, far = 10.

I've put the eye of the camera in (0, 0, -2.5)looking directly at (0, 0, 0) with up (0, 1, 0).

Suppose I want to place an object centered on z = -1. I want to find the X coordinate so that under this trunk appears exactly centered on the left edge of the screen.

I set up the following diagram to help me:

                                                                                                + z
|
 | /
-X<_______________|______/__________>X
 __ x__ | -one /
 | /
 | /
 l_ | __ /
 a | / | close to dist = n = 0.5
 | / |
| -2.5
|
-z

To find X I can use trig trigonometry simple. As tan a = l / n Y tan a = x / 1.5 then the formula for X is x = l / n * 1.5 = 2 / 0.5 * 1.5 = 6.

But when I use this value of x to draw an object to the left of the center, it does not appear centered at the edge of the screen. What am I doing wrong?