Heres what I do. I'll pull up the song I wanna sample in Acid (which has a beatmapping wizard). Then i put the down beat on a snare with a good, visible waveform. Then i put the other marker two snares away. (For like a soul sample its probably between 60-75 bmp). Then i just speed it to the tempo I want, chop, loop, or whatever, then add some drums. Hope that helps.
Dan