If you've got a very tight change, studying Google/Bing maps satellite views to find the exact location of the footbridge/s (or other relevant features) can help. The National rail station diagrams don't necessarily reflect reality, for example Luton Airport parkway on National Rail appears to show the two footbridges spaced out at about 1/4 and 3/4 of the platform length - i.e. one for the north end, one for the south (presumably because they've just cut off the north ends of the platforms beyond the canopies, where there are no facilities). The reality is that both footbridges are on the south half of the platform, so getting off where the "north" footbridge appears to be on national rail will mean a long trek down the platform.
This information was very useful a few weeks ago, I couldn't have made the two minute connection from Thameslink to EMR Connect if I hadn't been in a coach right by the more central footbridge.
Of course, you do need a reasonable idea of how long your train will be and where it's likely to stop, but with Thameslink 12 car trains it's pretty obvious it will use most of the platform.