Then again to back up what has been said the condition of the No 1 Mk III it may be a real gem or had a hard life which is pretty close for the 303's given the conflicts that have happened and to obtain a real mickey mouse one your either in the pipeline or have a deep wallet!
And as Alan de Enfield said loads have allot to do with it as the 174gn SMK has a different BC to the Mk VII projie also we do not use Cordite anymore so best as stated get to the range with a number of front sights and go from there as this web site provides plenty of info on what to do's given the depth of experience contained here. This web site provides a 25 meter ranging target with the appropriate elevation POI markings on it but as stated the modern projies flight will be different than the Mk VII projectile.

Factory rounds will have a POI differing from reloads and the variables and so on