for spectrophotometry if the absorbance change before and after a reaction is 0.001 would it still be accurate

It depends upon the S/N ratio of your instrument and the dynamic range. With a good instrument an amateur can measure changes on the order of 1E-5 au, so your signal of 1e-3 au is still large and credible. If at all possible, do the chemistry in the spectrometer so that the alignment of your sample cell is constant. To establish the validity of your result, measure the root mean square noise in the spectral region of your measurement and compare this to your signal. If the RMS noise is much smaller than your signal then the measurement is valid. (there are more sophisticated methods to extract information from noisy spectra that are beyond the scope of this forum). Next, repeat the experiment at least three times to establish that the result is reproducible.

Piece of cake.elementary my dear friend .dcpip in vc is tripped in malleil