> I think xntpd needs the clock to be within 500 ppm or the PLL won't 
> lock up.

Is that actually documented anywhere?

		case 1200:
			/* A factor of 1.1011010011100001 gives about 64ppm
			   error. */
			if (time_adj < 0) {
				time_adj -= (-time_adj >> 1) + (-time_adj >> 6);
				time_adj -= (-time_adj >> 3) + (-time_adj >> 10);
			} else {
				time_adj += (time_adj >> 1) + (time_adj >> 6);
				time_adj += (time_adj >> 3) + (time_adj >> 10);
			}
			break;