Is this a good design for a variable power supply?
You are using a variable resistor to alter the output voltage - this makes it a bad design especially if you wish to draw anything above a few milli amps from the circuit. Use an adjustable voltage regulator is my advice.

The variable resistor output voltage is fine if you don't load the output but as soon as you start to take current significant volt drops appear and you lose your output voltage accuracy. Also, variable resistors are not regarded as power devices so you can easily burn one out on even a few milli amps of load.