Corner positions for a subwoofer can be problematic and can cause unnatural peaks due to resonances with equidistant walls. It's best to place the sub at different distances from all walls. That includes opposite walls and the ceiling. Peaks cause by resonances can be used to optimize the sound but performing a mathematical analysis may be going overboard. Common sense also says to avoid equal distances from the nearest two walls and avoid equal distances between any wall, ceiling and opposite wall. If possible, avoid a distance from any wall that would reinforce the subs own internal resonance. And place it on the floor.