The stable commutator length formula for a family of words in $F_2$

Let $p,q,r\in\mathbb{N}$ and set $n=p+q+r$. Stable commutator length formula. Then

$$\operatorname{scl}(a^{-n} b^{-1} a^p b a^q b^{-1} a^r b)=1-\frac{\gcd(n,q)}{2n}.$$

The paper presents this as an unrelated conjecture at its conclusion. No resolution or partial result is supplied in the given text.