Shoe length = 31px = 11 inches = 27.94 cm
Mirror length = 690px = 621.9 cm
Mirror = 238px = 621.9 cm
Urek's back = 152px = 66 cm
2atan(tan(70/2)*(238/768)) = 0.291582821184 = 16.71 degrees
Distance = 21 metres
2atan(tan(70/2)*(152/768)) = 0.18700477227 = 10.71 degrees
Distance = 352 cm
Distance between Urek and the mirror = 17.5 metres
Urek gets out of the mirror's way before its light can reach him and repeatedly blitzes it, passing in front of it multiple times so he'd have to cross its diameter in the time it takes light to travel 17.5 metres.
6.22/17.5 = 35.55% the speed of light (Relativistic)
Bit of a lowball since Urek gets closer while blitzing it, but there's no way to properly measure that.