One thing I don’t like here is how a cappellas are categorized into a “range” instead of an “exact” bpm. I’ve given up on several tracks (mostly rap) because I simply cannot figure out the correct bpm. Loveshadow previously suggested that artists have an intro click track. Please do this!!!! You’ll get better results from remixers.

most acapellas do have an exact bpm, and if they have no bpm, check out some of the remixes of that pell.. should get you started..

I’m not sure what software/hardware setup you use, but in cases where I’ve had to guess, I’ve often temporarily trimmed a file to start at the strongest dB peak, and simply did some head-nod or lap-slapping to guess at a relative bpm, created a metronome or click track (if one doesn’t exist on your software, listen to few bars to check for drift, and adjust incrementally. Many rap pellas also have some warm-up type “uh…yeah.. uh-huh…” type of start-off, which can be a big help as to bpm. The tricky part can be deciding if the pella is coming in on the one, or the two, or somewhere in between… especially if there’s no chorus- and some artists get annoyed if the pella has been syched on the one, when they recorded it on the two, etc. Click-tracks would be a great way to prevent misunderstandings- or at least a small snippet of the original full mix, as a starting point. Sometimes pellas are fun just to get creative with, but often it would be nice to have straight-up synch/bpm clues, as to give a true “same-but-different” remix. Dunno if any of this blah-blah helps, but I thought I’d chime in; not everyone has beatmapping, and even then, sometimes that can hurt more than help. I understand the frustration, with lots of “dead air” or no reference whatsoever, pellas can seem to drift or wander no matter what bpm you try… then you feel dumb when someone else remixes it, or you hear the original- “Oh, THAT’s how it’s supposed to fit…”
At least I’ve done that, heh heh.
I’ve had more trouble with melodies than rap, simply b/c the rhythms of plosives seem more metronomic to me, whereas melodies can hold notes, be more randomly staccato, and can jar the ears of the artist, or listeners of previous remixes more it seems. OK, enough blah-blah, shutting up…

Alot of the times you will find the BPM if you click on the link of the song on the artists page(if they are nice enough to supply the BPM). Otherwise, I step away from pro-tools and use Sony Acid 6’s beatmapper to help me calculate the correct tempo. In this case, I simply listen to where the ‘1’ might start and continue through the rest of the lyrics in the manner. It usually works.

Hope this helps.