I read somewhere[so don`t take it as gospel]that the No4 Mk1 sight, each click was 1/2 in at 100yrds. While the No5 mk1 sight ,each click was 1/3 in at 100. The writer was suggesting that the No5 sight gave a more accurate adjustment for service rifle shooting. I have never tried this and have no idea what it would do to the scale at longer distances!