I am pretty sensitive to tempo variations and wondering if I'm being too obsessive about it. I'm working with a drummer who is a little green and, with his consent, I am trying to coach him in filling in some of the cracks with things he needs to work on. Tempo is a big issue; he's gotten much better at it but will still manage to speed up or drag in certain areas; usually these are sections where his playing will go from simple to busy or vice versa, so I'm trying to make him aware of that tendency, so he can work on it. He has an app on his phone that will show him his tempo as he's playing, and when he watches it, he manages to stay within what, to me, is a tolerable level of variance. But when he starts going beyond that level, I have to stop him and remind him to pay attention. I might alert him to that when he starts to approach a certain bpm or so above or below where we started, but I'm usually feeling it before then. I'm not saying what that level is yet, because I don't know if I'm out of bounds for what most people would say is tolerable, and I want your unvarnished views. Also, if you all agree with me, it could be helpful for me to be able to show him that I'm not being too picky. And if you don't agree, then perhaps I am being too picky, and that would be useful too. Editing to add, since I'm sure it will come up: He has practiced with a metronome for years and he's gotten very good at keeping in time with a click; this is a problem that he has in playing freestyle without assistance of that sort, so I don't think using a metronome or click at this stage is going to fix this.