The free lift is the extra lift that causes the balloon to rise. It is the lift after all assembly and ready to release. This is done when filling the balloon so the weights of the beacon, antenna and harness need to be known. The weight of the balloon itself is not necessary for this step as the gas will be lifting the balloon and it is the extra gas that is being measured. If one balloon is used, the math is simple. If two are used, do the same math but divide by 2 and do that in each balloon.
There are many small scales that can be bought. They are cheap and can be less than $10. There is always concern about the accuracy of the scale and if the scale has changed. It is a good idea to buy a low cost set of laboratory weights to verify the operation of the scale is correct. I use the set sold on Amazon as “American Weigh Scales 14-Piece Small Calibration Weight Kit – Red”. It costs about $13. You can check linearity and accuracy and have some peace of mind about the launch.
The usual method is to put a weight with a clip to hold the balloon. Gas is put into the balloon until it can lift itself and then a bit more. At any point, pause, role up the neck of the balloon just to keep the gas in and clip it to the weight. The scale will read a negative number which is the lift. (If it cannot display negative numbers, don’t tare and just do the math.) Do this in small increments as it is easier to add gas than to remove it.
The room needs to be still. The balloon will wave in breezes and just moving around makes enough to be seen. Allow the air to be still and that will be the free lift. The neck can be rolled up and clipped for a while as there is no pressure in the balloon and the gas will stay where you want it.