I seem to remember the usual figure is 5%
For large quantities such as castings then that seems far too much. Reducing to 2% would make sense. If you go on the low side it will normally kick the reaction off if you warm things a bit.
I would check these figures, they are from memory and I must confess I have never found ways to measure these small quantities I have been lucky with guesswork so far. I am sure I err well on the low side . I think most of the run away and cracking comes from erring on the high side.
Flux