Well before long people will chime in and say rely on your ears
. It's the truth though. If you are new that is one of the fundamentals to work on first. I relate it to dancing and feeling the rhythm of the music. The rhythm is your bpm's if you can sing a song and sing another song with the same rhythm or tempo then you are starting to understand bpm's. It is normal for the bpm to shift. You would only use bpm to give you an average not an exact bpm. You know if your bpm is 134...you can play something in the +/- 5 bpm range. So put something on around 130-138 and get your cue ready and use your pitch slider to adjust and then make fine adjustments until the mix is there and fade into your new song.
Next remember to listen to your mix and don't mix vocals from one song onto another and don't mix two different bass patterns....turn the bass down on the song you are taking out and let the bass play for the song you are playing next. Most important and once again we all will tell you is PRACTICE PRACTICE PRACTICE. It is truely the only way to feel and do what I'm trying to explain to you. Alot of people will read this and understand what I'm saying....maybe they can explain it better if this is kinda vague. But all in all hope this helps.