What is 156.300MHz plus 0.0005 percent?

Answer 1

0.0005 percent is 0.000005 or (5 x 10-6)

(156.3 x 106) x (5 x 10-6) = 781.5 Hz.

That's the margin you have for off-frequency operation.

If your transmitter is more than 781.5 Hz high or low, the transmitter is out of spec,
and you may be in violation of your license.

That gives you the slot between 156.299 218 and 156.300 782

Remember that if your counter isn't definitely also within 0.0005 percent, then you can't
trust anything it tells you. Even if it's in calibration, that only means it's within its own spec.
The counter still needs the same spec or better than what you're counting.