This is the procedure:
to each 500μl of standard (in the desired concentration)
1-Add 400μl 0.56M HCl
2-Add 150μl DNP reagent (0.2% 2,4-dinitrophenylhydrazine in 2N HCl) and vortex, approximately 5 sec
3-Incubate at 30°C for 30min
4-Add 1ml 2N NaOH and vortex, approximately 5 sec
5-Decant into cuvettes and read OD at 540nm.
If you use 96 well-plates then calculate the amounts to use based on these values. From my experience, plate assays are always more variable (pipetting errors, hard to mix, and so on).   _{Your Comment}