Ah. Hobby engineering. You have strayed into an area of intense interest. Here is a simple Rube Goldberg device which may give you other ideas:
build a short teeter-totter ("TT") with a laser pointer clamped to it, so that the laser points at a wall. Make sure the TT is fairly level, the fulcrum of the TT is firmly supported and that it is fairly balanced (so as to not add weight to your string). Attach one end of the TT near the free end of the string. Turn on the laser, make a pencil mark on the wall where it hits, load the string, and make your second mark.
Call the length from the TT fulcrum to the string = TS.
Call the length from the string to the wall = SW.
Call the length from your first to second wall mark = WW
Call the length of stretch of the string = SS
Then: SS/TS = WW/(TS+SW) ---> SS = (WW)(TS/(TS+SW))
If you build a TT with a 2 inch half-arm (TS), and your wall is 9 feet away, you will get a movement magnification of 110/2 = 55. This might not be enough.
Another idea would be to mount a small mirror on the TT and hit it with a fixed laser, so that it bounces back to the wall. This would double your sensitivity (movement magnification of 110). If you made TT for the mirror = 1 inch, you could get approximately double that (218). If you wanted to add another mirror at the target spot on the wall and make your pencil marks on the opposite wall... well... you do the math. :-) As long as you keep the angles small (less than 5 degrees) you can dispense with any trig and just use simple geometry.