Cyrille,
I think the scope should be as close to the barrel as possible. You have to have room for the sights on the revolver and for the hammer to be cocked. Sometimes it takes a hammer spur to cock the revolver, since you may not be able to get your thumb over the hammer. You want to be sure you allow enough for the scope bell if it has one. I don't remember seeing a bell on LER pistol scopes. I prefer 2X to 4X power scope. Any higher magnification causes too much movement to be shown through the lens.
You may want to ask your gun shop or sporting goods store to let you try it on your revolver to see how it fits before you purchase. Keep the end of the scope back from out in front of the gun to keep from blowing powder residue all over the scope and lens.
Hope this helps.
:)Deadeye Dick