I made this pic a while back to explain the formula I use to work out correct sample lengths based on BPM.
Reverse it and you can work out a BPM with a stopwatch.
60/2.526= 23.75x4=95 BPM
This is most useful for finding the BPM of acapellas that produce no transients for beat detection software to work with.
Apart from creating true loops that will run for days without drift you can also chop up the loop into small bits and use MIDI to trigger each small slice running from a sequencer grid or by tapping in manually, the added benefit of this is that you get to juggle the slices around if you want to.