...between AUTO DETECT and CALCULATE bpm/tempo. I heard that bm3 doesn’t do either really well so where is the accuracy? Also what might be the reason that 120 is the default bpm. I figured upon loading a clip it would tell me the duration and bpm based on that but I guess not 
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With calculate, you put in the number of beats, and the bpm is calculated based on the length of the sample. It’s 100% accurate as long as the number of beats is accurate.
As for the default bpm, what DAW have you ever used that doesn’t have some default BPM? 120 BPM is pretty typical.
You may have read an old post that said auto detect did not work well, this is no longer the case, I hounded and hounded @mathieugarcia to put in the calculate tempo option so that we always had a 100% accurate option, and when he had, he casually said "Oh yeah, i have made the auto detect a bit better too" needless to say, auto detect is near (Couple of decimals leeway, IE nothing) perfect about 99% of the time.
Where as calculate tempo has a small bug and can give the wrong results by reverting beats size, I have a sample here that calculate tempo calculates correctly then reverts to 12 beats instead of 16, so i have to manually enter tempo.
One or the other will get you where you need to be, but tempo train yourself so you know where abouts it should be (Much easier than learning keys etc)
Thanks, will do
Needless to say, both work really well, sans that extremely rare bug i found.
Auto Detect is working perfect - been timestretching long vocal passages Very accurate. No weird sounding artifacts and timing has been spot on