I usually put the mould on the scales and tare it, then fill the mould with water and note the weight. Times this weight by 1.25 and that's a rough estimate of how much solid M&P base it will take to fill.
Be careful about taking weights when M&P is melted because during the melting process it loses water and, therefore, weight. I use this method because it allows me to get a correct weight from the M&P out of the box.
HTH
Kevin x