Did you not get any instructions with your can.
Distance will vary depending on the manufacturer, its normally between 40 - 50mm, measured from the weld
See below, marked with the red arrow.
You want a good quality blade in a haxsaw or an angle grinder. Cover up your swing arm with something to protect it from over enthusiastic cutting / sawing, Also take care you dont cut your heel plates.
Oh and measure twice, cut once.