How can I estimate blocking window distance in an sCMOS sensor setup?

I am trying to establish an appropriate distance between a spectrograph output window and a CMOS sensor.

The spectrograph creates a focused image on the CMOS sensor but I am trying to restrict the light as the initial pixels are capturing light of higher intensity from 0th order wavelengths. Post processing is possible, but I would rather physically restrict the wavelengths.

I need to block out some of the pixels in the border. Therefore, I need to determine at what distance the blocking window needs to be placed.

I hope the image below will illustrate my problem.

The setup is:

1. Motor operated scanning mirror
2. Headwall VNIR Spectrograph
3. PCO Edge VNIR 2560 x 2160 pixels sCMOS sensor

I am trying to “see” 2560 spatial pixels and 500th-1600th spectral pixels. So, I am thinking the horizontal window dimensions would remain the same (at 16.64 mm), but the vertical dimension is unknown. For example, vertical dimension would be 7.15 mm if the window is placed EXACTLY on the sensor. I am trying to establish how far to place the window. I can place the window anywhere between 5 and 10 mm from the sCMOS sensor.