Starting with the simple stuff.
For the game of Warhammer 40,000 the normal play area is 6′x4′, more usefully in inches 72″x48″.
Most of the game activities revolve around actions that depend on distance (e,g, moving, shooting, assaulting) and these actions almost always have distances that are 6″ or a multiple thereof (e.g. 6″ moves, d6″ run, 12″ rapid fire etc). So, before the game starts, it is useful to imagine the tabletop in 6″x6″ squares (so the playing area will be 12 squares wide by 8 squares deep).
This will help gauge weapon ranges, how long it will take to get from point A to point B etc. It won’t give accurate results, but it will give you a ballpark, and will let you know if something is definitely impossible. It’s inexact because a) you’re guessing, and b) most of what you do will be diagonal – so the orthogonal distances that square counting gets you won’t be right.
To overcome A requires experience, good spatial reasoning, and some practice (ie set up a tabletop and guess the various ranges, measure and see how you did).
To overcome B I’d suggest the following – imagine the rectangle that has one corner on your starting point, and the opposite corner on your end point. Add the longest side of the rectangle to half the shortest side and you will get a good approximation of the true distance.
So when assessing the battlefield, consider the deployment zones, terrain, and objectives – imagine your 6″x6x grid and start to consider what you can do, not just in this turn – but right through to game end. Just as an example, say at turn 2 you want to decide which units to send to which objectives; it is easy to underestimate how long it will take to get there – the grid will let you know if reaching that point is possible at all within the time available. For example a footslogging unit walking on turn 1 in a dawn of war scenario will not be able to make it to the opponents edge of the table even if they go in a straight line with no obstacles – unless they forego shooting for running.
Before the game starts, and during every turn, look at distances and consider them in terms of game turns to get there, or ranges required to shoot there, etc etc. Look at your opponent’s position and do the same to gauge what he can do, and what he may be going for. Don’t forget to use legal measurement to check your estimates during the game, for example movement and range after declaring shooting (yours and your opponents!)
Now stop reading this, and go practice eyeballing the tabletop.