The general rule for gyro gain is to run your gain sa high as you can until you notice tail wag. Then bring down the gain until the wag disapears. A good starting point is where tony mentioned at about 65% up to 70%. The 450x is a sweet heli and capable of hard 3D. I hope you get a lot of enjoyment from it.
yeah 65 is GOOD... I flew my friend's 450x at 70.. but like helikat said. go up till it wags and back down slowly till it is steady. and you will be perfect.
Just for clarification, that "65%" is for the DX6i where each "%" is equal to "2%" on the 7s and 8. So 65% would actually be 30% on the 7s or 8 (15% above 50 on the 6i). I wish Spektrum would just stick with one setting for all of it's Tx's lol.