austra 1998 - ignore the previous post.
There is a limitation on the doughnuts (Brit spelling of donuts!) in a visual approach and they should not be considered reliable. My own preferred technique is to extend the centreline before turning onto the final approach. If it is marginal visibility or there is a real chance of a mis-identification of the runway due city features, long roads etc then I leave the autopilot in and push NAV on the FCU. (NAV thus appears armed in blue on the FMAs below TRK as I would be using TRK/FPA). That guarantees a turn onto the extended centreline - the first disaster of being locked onto somewhere other than the runway is thus avoided! In terms of vertical profile I have pre-entered the runway in use - say 'EGKK26L' or whatever in the prog page bearing/distance field. I then use 3xDist + elevation of the runway to get the correct position in space on a 3° slope. For example, at Gatwick (elev 200') at 4nm I would wish to be at 1400' (3x4+ 200'). For all practical purposes this will be good enough to get you very nicely in the right place at the right time. Hope that helps.