Weird value of track width with calculator
First of all as already commented to your question, you don't design a circuit according to what the power supply can deliver, but to what the circuit in design will draw as an absolute maximum (plus some safety margin). 

What I usually do, is to write a comment to each section of the schematic with the estimated current consumption. 
Add them up and you know your complete current consumption.
Even if you are for some parts of your circuit not completely aware about the current requirements you can usually find some information in the datahsheet, estimate or simulated (by something like LTSPice for example).
On top of that you want to add some safety and you have your maximum current your complete circuit will draw.

With this information you can either use a lookup table or one of the plenty trace with calculators around (e.g. http://www.circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/) and you get your minimum trace with. 

If space is not absolutely critical usually your PCB design software uses some default trace widths, select the next higher one to the calculated one an you are good to go.

Besides that various methods exist to increas the current capabilities of tracks, such as


using thicker copper (2oz instead of 1oz thickness for example)
add a wire connection
keep the solder mask of the high current tracks open and thicken the tracks by solder.