For sure Vbg with flaps is not equal to Vbg clean. If it's not published for your aircraft then the only way to find it is to go up there and measure it.
Set up the configuration you want to test, pull the power, hold a steady speed and time the descent over 500 feet (helps to have a passenger who can take care of this). Repeat for a range of speeds, separated by say 5 knots. Afterwards sit down with a calculator and work out the glide ratios, plot them and interpolate to find the best.
Tedious though makes for some interesting flying.
In my 182RG, Vbg clean is about 85 knots, Vbg dirty is in the 70-75 range.
Note also that the curve is pretty flat close to Vbg, so there's no point in obsessing to the nearest knot.