In casting there are many variables, the secret of good casting is getting these variables under control.
If possible use electronically controlled melting pot (these are more accurate than thermostat types which can have large ranges of temperature between the cut in and cut off of the heating elements)
Select a temperature guided by your manufacturers recommendations use the lower range for bulky and easily filled items and higher ones for the thinner more difficult ones. Adjust the temperature up or down only after careful consideration of the results achieved. You will get better results and be able to cast at lower temperatures if you vent the mould to allow quicker easier filling.
Always try to pour in a similar way, when you remove metal from the pot it is losing heat the longer the time before pouring the more heat lost therefore a big source of problems and variable results.