Record your performance, & use that to improve. Measuring live bpm is viable, but to me, it has little value outside of curiosity. Keeping good time is all about listening, then internalising that pulse. In a live performance context, relying on a machine to tell you if your time is out isn't helping you appraise the important factors. In most genres, a bit of push & pull is desirable. It builds tension/propagates release/adds excitement/delivers mood. It also makes music real. Personally, I see little value/pleasure in seeking to reproduce a perfect studio take on a live stage, otherwise, what's the point in live music! It needs to breathe, but that's no excuse to be sloppy. In the end, it's all about the vibe, & if a song ends up 5bpm faster at the close than it did at the start, but the vibe is superb, then as far as I'm concerned = job done!