LTC3106 solar harvester example - different resistance values but same ratio
The relationship is simply a voltage divider:
$$ V_{RUN} = V_{IN} \frac{R2}{R1+R2} $$
$$ \Rightarrow  V_{IN} = V_{RUN} \frac{R1+R2}{R2} = V_{RUN}(\frac{R1}{R2}+1) $$
One criteria for the value of R1 or R2 is such that the current through it is much larger the current through the RUN pin to control error. For example, looking up from datasheet, on page 4, RUN Input Current = 10nA max. If you want to cap the error due to RUN Input Current variation to no more than 1%, approximately then:
$$ I_{R2} \gt \frac{10nA}{1\%} = 1uA$$
$$ R2 \lt \frac{0.6V}{1uA} = 600K\Omega$$
You do want the resistance to be high to save power. So going with R2 of around 365K like in the datasheet example (or 600K) should be good for most cases.