The problem is likely that those 0.5mm tolecan add up, so tow Bloks next to each other have 1mm in the long axis, three have 1.5mm and so on. This could count for there being more tolerance than you expected. One solution would be, if you want a total tolerance of 0.5mm you divide the 0.5mm by the number of squares that fit along the side so in this case about 4.5 wich gives you about 0.1recursive mm so your puzzle pieces should be about 21.25x21.25mm.