Reaction mixture gives very dark color. If not adjusted abs of 0.7 at 734 nm, you can not get reliable result. 734 nm  is wavelength give the max absorbance of this mixture.

Any real measuring instrument has a limited range over which it can accurately measure absorbance. Many instruments will become non-linear and fail to follow the Lambert-Beer law, approximately from 2 AU for this reason the reading of absorbances were conducted in a condition which give the best (linear) response from the used instrument. 

You can find an answer in this article. 

Good Luck