Air density formula:
ρ(h) = ρ0 * exp(-curve * h/h_max) - ρ0 * exp(-curve)
And this is what i found experimentally:
v=53,7541*ρ^(-0,2457)
v: velocity
ρ: air density
What this empirical formula says that if your rocket is at a point where air density equals to ρ, rocket will heat. For instance...