New tyres have 8mm of tread and if you take away the legal minimum tread of 1.6mm that gives you a legal usable tread depth of 6.4mm which means a half worn tyre will technically have 4.8mm of tread.
Most tyres start with 9mm. The SportMaxx tyres fall off a cliff at about 3.5mm, so that makes useable life of 5.5mm, so as you can see, straight maths gives 6.25mm. However, due to reasons I can never grasp, once wear sets in, it accelerates - in same way if you have a suspension fault that causes uneven wear, even once fixed, the wear is more excessive on the worn patch.
This is why I always say that 5mm part worns are not financially effective, especially when most places want £15 to fit one.