What you can do in the case that you know what the distance between your objects or fixtures needs to be, is use the duplicate tool. Here you can specify the distance in between.
If you have a set of fixtures that are already spread evenly, but need a bigger or smaller gap between them, you can select them all and grab one of the corner handles to change the distance. You can hold shift while doing so, to lock the direction of the resize.

Thank You heaps.

that is indeed a helpful hint.

Adding to what Floorian said let's assumes that you want the first and last fixtures to be 13.45 m apart and you're going to add 7 copies, using the X-axis as a reference and viewing from a top view.

Select the original fixture and to duplicating do the following: Duplicates: 7, Offset X: 13.45/7, that is, the total distance divided by the number of copies.

Bingo!!!

If you haven't placed the first fixture at x=-6.725 and now want to center them in relation to the zero axis: group them together and then change the x-coordinate to 0. Separate them.

Guess add an option to "force" mesure on spred even should be a nice idea