To use simple scaling to taper something like a column, you can pushpull a cylinder as in the first example tut, then select the top circle, scale it by pulling a corner handle WHILE you are holding down the Ctrl-key, this is so the centre-point becomes the scaling origin, then pull it in a little inwards, THEN type 0.5 in the VCB to make the original 800mm into 400mm [OR type 400mm - note units suffix]...
The two tuts below and this scaling way are all pretty easy and straightforward...