@Pilou : well, one can't say "why don't you do x ?" to someone who's precisely asking how to do x ...
Edit : Ha, nevermind, I see you've added steps now.

@mk : Based on your initial description is seemed that you wanted to move an object along a specific direction, of a certain amount of units - which is done either by first drafting a regular line to help you with it, or, as described in the steps I outlined, by defining the desired vector and reference points using the click-and-drag helper lines.

But now it seems that you want to *place* and object at a certain distance to another. So for that, you need to either draft a line that will specify the desired distance ; or, use the realtime helper lines (and more particularly, the points defining them) to establish this distance. To do so, you need to :

> Initiate Move
> while move is in idle, click-and-drag from a point on your target object, and move the mouse in the direction the target point will be (in your case, horizontally)
> type in the desired distance. This will put a little dot at the target distance, and also an End reference point
> to be more safe, mark this end point by placing another helper line. Now it will be impossible to miss
> this is now your target point to snap to for the move

As shown here in two scenarios :

https://www.youtube.com/watch?v=VE_Vr7YquEE

Arguably it would be much clearer if the End points were actually shown on screen, as opposed to being hidden but snappable. If that was the case I would personally use this method much more often, but I must say that I always end up drafting an actual line, because relying on hidden dots is way too dangerous and caused me to incorrectly draft things too many times. Ideally both the typed-in distance as well as the end point should both show on screen. Perhaps there is a way to do that already, but I don't know how.