Sorry londonblue, JT is correct.
When weighing an aircraft - like pb84's Maule for example - to calculate the empty CG location, you measure the weight on the mainwheels and the weight on the tailwheel (with the fuselage levelled as per the factory instructions).
You then calculate the mainwheel moment - mainwheel weight x mainwheel arm (from datum) and the tailwheel moment - tailwheel weight x tailwheel arm.
You sum the two weights to get the empty weight. You then sum the two moments and divide by the empty weight, to get the empty CG arm.
There are important details in this process, for example, you must do the weighing in a closed hangar to avoid any wind-induced lift affecting the scale readings and with a taildragger, the scale used for the tailwheel must be more accurate than the one used for the mainwheels, because the weight is significantly less and so any error would be a bigger percentage.
Interestingly, the location of the datum does not matter. As long as you pay attention to the signs of the arms, the empty CG location will come out correctly with any datum. For example, you could use the mainwheel axle as the datum, in which case the mainwheel moment is zero and only the tailwheel moment would be used in the calculation.