Voltage Division with coarse and fine adjust

In designing the Voltage source of the last post, I originally wanted to have 2 adjustment knobs, one coarse and one fine. I concidered designs using two linear potentiometers but the fine adjustment wasn't as I wanted it to be. I wanted it to always provide a uniform range +- around the coarse setting ( I call this +- range delta V). It didn't provide this, instead it would give a large delta V when the coarse was set at one extreme and zero delta V at the other. Circuits A and B below demonstrate this effect.
I then discovered that by using a standard linear pot for the coarse adjustment and a dual gang linear pot for the fine adjustment my desire could be accomplished as represented in Circuit C.

Equations for Vout/Vin for the three circuits and corresponding plots with R1=5k, R2=100k, R3=20k are shown below. I've introduced variables x and y to represent the fraction of R2 and R3 which are enabled by the pot knob and can take values from 0 to 1. The plots show the extremes of the fine pot (y=0, y=1) for all values of x. The size of delta V can be set primarily with the value of R3. I do like this design but didn't end up using it because I was impatient and didn't have immediate acces to 5k dual gang pot. Maybe next time!