Since you are changing the FOV and your reticle is not centered it is always going to move like this. If the FOV change is constant, then you can just adjust the reticle position in your widget until it is correct when zoomed in. Now you have your original and your zoomed in positions. When you switch FOVs send a message you widget blueprint to switch positions as well, then when you zoom back out you switch back. If the zooming is animated you can use a widget animation to make the switching smooth as well.