I'd expect atleast 350mm RHAe in the center if we're talking 420mm composite + 240mm solid titanium (660mm LOS).
The edges of the mantlet area could present as high a value as 400mm RHAe considering the 240-280mm RHA turret armour behind the 420mm mantlet:
PS: Went and sat in a Leopard 2A5DK today and had the opportunity to eyeball the thickness of the add on armour on the upper hull. These blocks of armour looked atleast 5cm thick, probably more. So the upper hull on the newer leopards is very strong. Also had a good 5 min in the gunners position, and I must say the gunners optics are excellent, extremely crisp (no LCD screen for the gunner in the tank I sat in though). Also noted a small joystick to the right of the gunners control handle, not sure what that was for?