Stereo Speakers - How far apart?

[cfact]

@sebastian - what is that 45% number based on? With subs it is not as simple has dividing by about two - if I hear "I am self-confident" - my subconscious can take that in whatever ear it comes through, or whether mono or stereo! So genuinely curious what the downside of mono is... a 55% drop in efficacy seems too much.

My bluetooth speaker can additionally be plugged in (i.e. no need to transmit audio via bluetooth), so I don't think there should be a lossy issue.