I will show $\mu = \tfrac{1}{\log 4}$. 

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I first prove the upper bound $\mu \leq \tfrac{1}{\log 4}$.  Fix a positive integer $r$. For $0 \leq k \leq r-1$, let $N_k$ be $2^{k/r} N$ rounded to the nearest integer, where $N$ is a large parameter. Considering the $N_{r-1}$ intervals present at time $N_{r-1}$, let $X_k$ be the number of them created between times $X_{k-1}$ and $X_k$; with $X_0$ the number created before time $0$. Then
$$X_0 \frac{\mu}{N_0} + X_1 \frac{\mu}{N_1} + \cdots + X_{r-1} \frac{\mu}{N_{r-1}} \leq 1.\quad (\ast)$$

Of the $N_k$ intervals present at time $N_k$, at most $N_{r-1} - N_k$ can be destroyed by time $N_{r-1}$. So at least $2 N_k-N_{r-1}$ of them survive until time $N_{r-1}$. This means that 
$$X_0+X_1+\cdots + X_k \geq 2 N_k - N_{r-1} \quad (\dagger),$$
with equality for $k=r-1$.

The coefficient of $\mu$ on the left hand side of $(\ast)$ is linear in the $X_i$, so it is minimized if we take the $X_i$ at a vertex of the polytope cut out by $(\dagger)$ and $X_i \geq 0$. A little thought shows we want the vertex where $(\dagger)$ is equality for all $k$, so $X_0 = 2 N_0 - N_{r-1}$ and $X_k = 2 N_k - 2 N_{k-1}$ for $1 \leq k \leq r-1$. We deduce
$$\mu \left( \frac{2N_0-N_{r-1}}{N_0} + \frac{2 N_1 - 2 N_0}{N_1} + \cdots + \frac{2 N_{r-1} - 2 N_{r-2}}{N_{r-1}} \right) \leq 1.$$

We plug in our optimized values:
$$\mu \left( 2r - r 2^{(r-1)/r} \right) \leq 1$$
so
$$\mu \leq \frac{1}{2r (1-2^{-1/r})}.$$

Finally, taking the limit as $r \to \infty$ gives $\mu \leq \tfrac{1}{\log 4}$ as promised.

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Now, for the lower bound. Again, fix a positive integer $r$. Divide the circle into $r$ equal arcs; each arc will be subdivided into intervals. At any point in our process there will be some integer $k$ such that one arc is divided into some intervals of length $2^{-k/r}$ and some of length $2^{-(k+r)/r}$; the other $r-1$ arcs will contain intervals of only one length, namely $2^{-j/r}$ for $k<j<k+r$. (Obviously, I am ignoring rounding issues.) As time ticks on, we subdivide the intervals in the first arc until they are all split in half, then move to the next arc, and so on. 

This achieves (again, ignoring rounding)
$$\mu = 2^{-(k+r)/r} \left( \frac{2^{k/r}}{r} +\frac{2^{(k+1)/r}}{r} + \cdots \frac{2^{(k+r-1)/r}}{r} \right) =  \frac{1}{2r(2^{1/r}-1)}.$$
Again, sending $r \to \infty$ achieves $\tfrac{1}{\log 4}$.