It's so variable that it's basically impossible to say, it entirely depends on your max heart rate and your current threshold HR. Gotta do a LT test while fresh and snag the HR from that. The other thing that makes it tough is if you're even a bit tired your HR doesn't respond the same as it does when you're fresh, so to do tempo you in part need to remember what tempo feels like when fresh and go off PE if your HR is being funky.