How to measure weight at end of arm
You will never get the kind of accuracy you need detecting the deflection in the arm like that.

Instead modify the clamp. Perhaps something like this.



The longer you can make the clamp block, the better the accuracy since a longer block lessens the effect of manufacturing tolerances between the hole and the sense point. Though you can likely calibrate that out. 

Of course you also need to make it long enough that 100kG on the other end does not swamp or crush the force sensor. I guess the ultimate length should get you just under max sensor force at your max arm load.