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Let $p$ be the number of pie crusts, and let $f$ be the amount of flour per crust. Because the total amount of flour needs to remain constant, we can express the relationship between pie crusts as $p\cdot f=c$, where $c$ is a constant value.

Since we know that 30 pie crusts each use up $\frac{1}{6}$ cup of flour, $30\left(\frac{1}{6}\right)=c$ or $c=5$. When $p=20$, the equation becomes $20\cdot f=5$, or $f=\frac{5}{20}=\boxed{\frac{1}{4}}$