Optimal boxes

Cut out four congruent squares from a cardboard sheet, so that the remaining part can be folded to create the box (with no top) having max volume. Use the sliders in the construction to set the lengths of base and height of the sheet. Move the green point on the sheet base to modify the cutout area. If x is the length of the side of the cutout square, what's the maximum length of x? (You can also move the green point at the sheet top right vertex to change its perspective view.)