KJLC recommends that a crucible liner be filled somewhere between 2/3 and 3/4 full when beginning deposition. The melt level should not go below 30% at any time or there becomes a risk of the beam striking the crucible, causing it to break.
For help determining how much material is needed, please see our E-beam Crucible Fill Rate Calculator. Fill in the volume of the crucible liner (in cc), density of the material (in g/cc), and percentage fill rate to calculate the amount of grams required.