Forky is right. The drag curve is a quadratic eqation and you want to know where its at a minimum.
As J3Pipercub says - If you really must find a number then go for a fly
Hope I don't sound patronising... but you would have to set up S&L with your approach flap, gradually reduce power and trim to maintain height until speed slows to a point where an increase of power is needed to maintain height and speed.
Remember speed for L/D max depends on weight, so reversed command will begin at higher speeds if heavier
