Calculating resistance for variable voltage
It's certainly the simplest way. Don't worry about the current draw -- even at 20V you'll only draw 0.1mA:

\$ I_{FB,max} = V_{out,max}/R_{FB} = 20V/(10k\Omega + 190k\Omega) = 0.0001A \$

When V_out is 3V you'll draw even less current -- about 15uA.  This isn't much at all -- just the price we pay for feedback.