Double check whether your center spot actually allows LATCH. Many vehicles only allow it on the two outboard seats and do not permit borrowing the inner anchors for the middle unless both the vehicle and car seat say it is okay and the spacing matches the standard but then... if the center is not approved for LATCH, install with the seat belt there and that is fine. Set the recline first, use the level indicator, then attach lower anchors and pull the strap tight while compressing the cushion where the child sits. A rolled towel or pool noodle to help achieve the proper rear facing angle is sometimes allowed but only if your car seat manual says so which, yeah... one inch or less of movement at the belt path is the goal.
