This website may help you to understand the variables affecting DO solubility in water. In my experience, I would recalibrate the DO meter and check the solubility under your conditions.

http://antoine.frostburg.edu/chem/senese/101/solutions/faq/predicting-DO.shtml

grrun

You need to make sure that your samples are at 20 C before getting the initial DO.  SM 21st edition does not give a specific number for the initial DO, but says "Samples containing DO concentration above saturation at 20°C may be encountered in cold waters..." [5210B 4. b. 4)] So you get an upper limit of whatever saturation may be at your temperature and pressure.  I think 9.15 is saturated at sea level pressures.  Depending on your DO meter and preferred significant digits that could be 9.1 or 9.2.  To reduce supersaturation, get the sample to the aforementioned 20 C, half fill a BOD bottle with the sample, put the stopper in, and do your best dance club moves (in other words, shake it up).  Don't worry about undersaturating.  This is a purely mechanical equilibrium and you won't be able to overshoot.

As far as a minimum DO requirement goes, we always stuck with 7.00 mg/L.  The dilution water has a requirement of at least 7.5 mg/L (SM 21st 5210B 5. a.)  We decided that approximately a 5% reduction was acceptable and rounded that to the nearest whole number.  Basically you just want to ensure that you have enough DO to allow for appropriate depletion.  You have the minimum depletion requirement of 2.0 mg/L, which pushes you down to 5.0 if you start with 7.0.