The output is roughly 50% duty cycle only when R1 = R2 (in standard circuit). Many calculators don’t show or calculate duty cycle.
Let's select a common ceramic capacitor: $C = 100\textnF$ ($0.1\mu\textF$). 74hc14 oscillator calculator
413 k$\Omega$ is within the acceptable range (below 1M). However, a common 5% value is 390 k$\Omega$ or 430 k$\Omega$. Use 430 k$\Omega$. The output is roughly 50% duty cycle only
that allows you to input specific voltage thresholds from your datasheet. Buffering: 74hc14 oscillator calculator
Only works for the single-capacitor, one-inverter RC oscillator. Doesn’t cover crystal oscillators or multivibrators with two inverters.