Designing a Power supply unit
What current will be used is determined by the load, not the power supply. We've had a number of questions like this before, like "how do I limit the current of my 5A supply to 1A".  

What your device will need is just the addition of the current for the different components on the board. You usually start with the power-hungry parts: motors, relays. For a microcontroller you'll have to look up in the datasheet what it needs at a given clock frequency.  

If you have a lab supply which displays current you may use that as a rule-of-thumb, but you may want to give it some headroom. Used current is definitely useful if you want to design a power supply.