Measure the pigment band of the solvent whose Rf value you've already determined. Since pigment bands are colored by definition, you'll be able to see each clearly, regardless of whether or not the solvent has disappeared. Say, for example, you had a solution that traveled 3.5 cm.
Divide the length of the pigment band by the Rf value to get the solvent front. If the Rf value for your solution that traveled 3.5 cm was .85, you would calculate your solvent front as follows: 3.5/.85 = 4.12 cm.
Use your extrapolated solvent front to calculate Rf values for solutions whose Rf values you failed to calculate before the solvent front dried up. Measure each pigment band, then divide by the solvent front.