If we use the calculator linked below. With "a" as the height above the MLP "C" (assuming C is set roughtly 4' off the ground. "b" is the distance fore and aft of the MLP "C". Then <A is the actually angle of overhead/height speakers to the MLP.
To be anywhere near 45 degrees then "a" and "b" would need to be the same, say 3' fore and aft of the MLP and 3' above (7' ceiling). In a 12' ceiling room with 2' fore and aft with the seating 4' off the ground we get 75 degrees , ie roughly the +/- 15 degrees of <B off vertical.
My heights are roughly 2.5m off the ground, minus 1.1m (4' off the MLP head height),1.4m above "C", "a". My distance from the MLP ("b") is roughly 2.5m. Consequently the total distance to the speaker is 2.87m and the <A angle is 30 degrees.
https://everydaycalculation.com/right-triangle-calculator.php
If I got mine nearer the 12' ceiling in the same location then my angle might get closer to 45 degrees. Are you calculating from the ground or ear height? 18 degrees just seems a bit low and if they are behind the Front L & R then you are looking at heights too.