How to get Affected FoV with BOIII client

Load into a costum match, equip a SMG with an ELO sight
Open the console by pressing ^ or ~ depending on your region
type "cg_fovscale 1"

Set your desired FoV in the settings.
Look arround on the map and find an easy see reference point you line up one edge of the screen with. From now on do not move or look arround.

Set your FoV to 75. At low FoV weapons zoom in less. 65 doesnt zoom at all, 75 matches the affected setting, 80 or more zooms a lot.

Because the whole image is zoomed in now, we use cg_fovscale to increase the FoV until it matches your prefered one.

If you like a 90 FoV and your ingame FoV is 75, use "cg_fovscale 1.31". If your desired FoV is higher, increase the FoV scale value until you're able to see your reference point again. You can use decimal points to match it exaclty.

I don't know if this work in the base game and if console commands can get you banned. Try at your own risk if you don't use a different client.