Stanford Advanced Materials recommends 65%-80% fill rate for evaporation materials. The Fill rate exceeds 80% will make the material to spill over and cause an electrical short, the crucible may be cracked. When the fill rate is too low, the melt level will be too low to make the bottom wall of the crucible break.
The fill rate means the material is fully melted. Most time, the crucible liner need to be loaded several times, pre-melted, and topped off to get the desired melt level/fill rate. Do not load more than 80% of the height of the crucible liner.