Today's weight/gross weight, find the square root of that, multiply that by the published Vspeed, and you have the new Vspeed.
For example at 750kg, Vs=45
For Vs at 680kg divide 680kg by 750kg and find the square root of that, which would yield 0.952. (You can think of this as a percentage too). Multiplying that by 45 we get a new Vs of 42.8 knots.
As you can see the difference is often very minimal, and not worth the effort
