grid alignment problem

ok im having problems using the grid alignment tool i can never get it to do what i want no matter the grid size i use, what happens is it snaps into place one way but is off another way or if i turn off the alignment tool and line it up by eye it looks all good but as soon as i click to save the timber where i want it it jumps off alignment grrrrrrr i have real ocd when it comes to this and its driving me crazy i end up with gaps everywhere and trying to redo things like 100 times and dont get anywhere. what i think is the problem is it aliens from the center of the grid line witch works most the time but not all the time, the other thing that would help is some sort of ruler system on the preview image