Resolution doesn't matter per se, but aspect ratio does. If you use a 16:9 resolution the calculator will assume your entire monitor is covered by an FOV of 106.26. This is not the case if you stretch 4:3, your entire FOV is then 90 which is why you must enter a 4:3 resolution.
For 360 match it's doesn't matter, but if you match ADS or scope to your stretched resolution using monitor distance it will make a difference.