If you specify the maximum current, then the trace widths will be calculated to suit.
If you specify one of the trace widths, the maximum current it can handle will be calculated. The width for the other trace to also handle this current will then be calculated.
The calculations are valid for currents up to 35 A (external) or 17.5 A (internal), temperature rises up to 100 ºC, and widths of up to 400 mil (10 mm).