Depending on throat design, it should be looked at like this.
Absolute MINIMUM CLEARANCE IS .003", regardless if custom, semi custom or factory. If a carbon ring develops due to throat design, then this tight clearance WILL pose problems.
I have my reamers ground for a .004" clearance and choose my brass accordingly. I have a few turn neck chambers and I still keep that .004" buffer.
I also reduce the clearance on all reamers from .040" at chamber end back to .020" max or .015" minimum. This reduces the chance of carbon rings due to keeping the brass long. Have 2 A191 chambers in 300WM that have the chamber end only .010" over max brass length, not trim length. This seems to work very well for me.
SAAMI chambers generally have between .005" and .008". Most I have measured were around .006" with the occasional .005" thrown in. This is ample neck clearance.
Cheers.