The ratio of Rf/Ri needs to vary between 1 and 20 for each input. This may be accomplished, for example, by using a feedback resistor of 100k and an input network of a 5k resistor in series with a variable resistance (potentiometer) that can vary from 0-95k.
You may be satisfied with using a 100k trimmer potentiometer which is typically available; full-range variation will yield a variation of c from 20 to 0.95 (only a 5% error on the low side).
The low-end stop of the range may be improved by putting 1% 1.91M resistor is parallel with the potentiometer...
(You may find many tables of "standard resistor values" on the web. Another one!)