Take pocket japanese and a clock, press 1+ and press = for a minute on every 1/4 note. There, beats per minute. same for half a minute, just x 2. lol!
In any case, it takes skills to be a dj, even when engaging a performance that includes your own productions to be mixed. In a simple solution for this, the mpc has a Tap function hehe. But when it's too digital to perform, simply just multitrack mix the shit if you lack the skills. It only takes a timestretch, pitch and Cooledit to mix.
Or, Finalscratch if you want to spend the money.