How do I find Rf values & length size of paper in chromatography?
$$\Delta R_f = 0.08$$
Let the separation distance be $\Delta r$ = 1 cm and let $l$ be the length of the paper in cm.

$$\Delta R_f = \frac{\Delta r\, \mathrm{cm}}{l\, \mathrm{cm}} $$

Solve for $l$, obtain $l$ = 12.5 cm.