https://support.office.com/en-us/art...9-873614413ce1
Yeah I don't think I understood the question the first time around.
This should be what you're looking for. Though, it's a graph not an arrow.
Alternatively, you could take the difference of the numbers, they're within you're steady parameters you conditional formatting to have a even sign. If it's negative outside your parameters, down arrow. If positive outside your parameters, up arrow.