Using lazer level to make line behind garage

I want to add an addition on to my garage. I want to continue the side wall in a straight line behind the existing garage. In the past when I have made additions like this I would run a string along the wall from front to back. connected at the front and when the string touches in the back that would be the line to follow. This gives a crude alignment but it's not the most accurate, and in this case the garage has 3/4" trim boards on the corners which will make this method pretty hard to use unless I remove the trim, which I prefer not to do until the addition is framed in.
I understand these lazer levels can be used for this same purpose, but I am not sure how. For one thing, I can not measure at ground level because the level would be against the concrete, not the garage wall, and that concrete is not exactly even along the edge of the wall. In some places the wall is one inch overhanging the concrete, because the concrete was never poured exactly straight. Therefore the level will be at least one foot above the ground.
The plan is to build this addition in pole barn style, so I want the post exactly in line with the wall. Or actually it will be placed back 2 inches from the wall to accomodate the thickness of a 2x4 plus the plywood 1/2 inch siding.
How do I do this? I should note that I will be doing this without help.
Thanks
LM
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On Mon, 08 Jun 2009 15:13:10 -0500, snipped-for-privacy@invalid.com wrote:

What's wrong with string? It'll give you far better than crude alignment.
You say you know the thickness of the corner trim. 3/4". You say you plan on using a 2X4. 1-1/2" You say you plan on using plywood. 1/2".
Add that all together, run your string line and mark your point with a plumb bob. Move your point in 2-3/4". Pat yourself on the back and have a beer.
G.S.
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On Mon, 08 Jun 2009 17:33:33 -0500, Gordon Shumway

Ditto the egyptians built the pyramids with string and sticks and they are virtually perfect.
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