I didn't have an issue, it calculated ~109dB as the average in the 300-1000Hz region, thus the -6dB point is~103dB, which is ~43Hz.
Yep, the -6 dB point is 103 dB, and Loudspeaker Explorer agrees. But then Olive says:
LFX is the log 10 of the first frequency below 300 Hz in the sound power curve, that is -6 dB
Problem is, it all depends on how you define "first". In Loudspeaker Explorer I interpret "first" as "rightmost", as in, first below -6 dB while descending from 300 Hz. Remember, it's the very definition @bobbooo suggested we use. Specifically:
And if we go by that interpretation, then the -6 dB point is not at 43 Hz… it's at 189 Hz!
I'm curious as to what formula you're using if it's not affected by this problem. I'm guessing your formula goes for the leftmost frequency that's above the 6 dB point, then use the previous frequency point right before that one?