^^^ This ^^^
I highly recommend using a hotplate and preheating your mould on it while your pot is heating up. I suspect your mould has not been properly preheated and that is why you've had to run your melt temperature so high.
As Goodsteel said, around 450 degrees on your mould and around 700 degrees on your melt should be good starting points and should be pretty close to where you want to be.
Hope this helps.