Before ATK bought out Speer and Alliant, the 2002 Alliant powder guide listed 6.4 Unique MAX, with an unknown 180 JHP, giving 1,065 FPS at 33,800 PSI.
A 10 % reduction starting load would be 5.8 grains.
I have long used 6.0 with a 175 lead cast TC bullet.
7.0 with a Rem 165 Golden Saber.
Both kick harder then a factory load, but seems fine on pressure.
I am not a believer in pushing the .40 S&W to max pressure with used brass, at least in my 1st. Gen Glock.
So, if it were me, and not you doing it with a 180 XTP?
I might try 5.5 starting and work up to no more then 6.0 MAX.
Disclamer: Not tested, and I'm not liable, cause it was me, not you, doing it!!
BTW: An early UMC 180 TC-FMJ factory load used 4.5 grains of unknown powder.
rc