[JoeGuardsman]

Have you ever been caught in an assault that was theoretically impossible yet the models are right there and in range? or Wonder why more or fewer models were hit by a blast marker. The problem is visual perspective.
It is a simple concept where your own personal eyes in your head line of sight distorts the position of the model, measure or blast marker. Movement has the largest effect on the game and inaccurate movement can cause the most problems. Example: If you hold your tape measure above the head of your model the downward angle that you can draw from your eyes through the head of the measure to the board will be some distance from the actual end of the movement. This can be exacerbated with tall and obstructive terrain that raises your arm and measure up even higher. Solution: It can be difficult at times but try to get your head above directly above the start and end point of your model's movement and keep the measure as close to the board as possible.
The other problem with perspective is blast markers. If you look diagonally down at the blast marker your perspective stretches it away from you. This could cause more or fewer models to be covered. This is also easily solved by getting your head directly over the template. Good luck keeping the template in the same position as you move your head around and count up your full and partial hits. The other perspective problem is if you hold your blast marker too close to your face and too high up from the models. The closer the blast marker is to you and the further away it is from the models the bigger it becomes in relation to perspective. I play Imperial Guard and on more than one occasion I've had to tell people to bring the template down so that they didn't hit as many people.
Those are my thoughts on perspective. Now go out and have some carefully measured gaming. Unless you play really friendly games and just don't care. I'm fine with that.