Adjustable SMPS using MC34063?
It is fairly trivial to make a power supply adjustable - as you said, the voltage feedback divider can be modified to give you a range of output.

You have to decide on what adjustment range you want to have, and do calculations at both the maximum and minimum voltages to make sure the power train is capable of working.

If you're designing a buck, for example, the highest voltage you want will dictate the voltage rating of the output capacitors, rectification and series transistor / MOSFET; the lowest voltage will dictate the minimum duty cycle and peak current capabilities of the inductor / MOSFET as well as the capacitor ripple voltage.

The exact point at which you're operating dictates how much conduction loss and switching loss you'll have - moving the output up and down will change the losses. 

So, in a nutshell, designing for the "middle" of the range is not a valid approach. You need to understand how your intended converter works and design such that at the worst corners of input and load, your devices are in safe operating areas.