Fig 1: The gain is set by the ratio of Rf/Rin. You are using the inverted (-) input, so your output is inverted so its -Rf/Rin. Rf is the net resistance between the output and the (-) input so figure out the equivalent resistance of the top rheostat and the series resistance with Rrheo at 0 and at max. Now calculate the equivalent resistance of the top resistances in parallel with the bottom resistor. This is the minimum and maximum feedback resistance, or Rf(min) to Rf(max) . Rin is your R1. The relationship between Sp/Se is -Rf/Rin. You have a low value for Rf and a high value for Rf, so your relationship is a range from -Rf(min)/Rin to Rf(max)/Rin.
Fig 2: is similar, gain = -Rf/Rin. Rf is the rheostat's value from 0 to max ohms. Rin depends on the frequency of the incoming signal because of C1's reactance. At DC, C1's reactance is infinite, so your Rin is infinite. At very low frequency, Xc is very high so Rin is R1 + Xc approaching infinity. At high frequency, C1's reactance approaches 0, so Rin is ~ R1. So depending the Rheostat and the frequency, you'll get a couple of different answers.