Is that an NG? My memory says 280/ M0.73 for a 737-300 - no worries these days as with the CI we fly, cruise is about M0.73 and we descend at 250
Our Part B manual says the above speed for clb/ dsc and whatever turb N1 gives you in the cruise. It's in the Supplementary Procedures under Severe Turbulence. What does your manual say?