Reply to knowing the right tempo

how do you know when you have the right tempo? is there a way to tell or an instinct?

I've been beatmatching for a month and I can get the songs matched for a good 15 to 20 secs but if I leave for a minute the songs are usually off. is there a way to set the.right tempo from the start?

also, roughly how much do you usually move the tempo control? I can't tell if I'm over, under or accurately adjusting it. I use 6% on the cdj900's


thanks!