An IP gauge may be off by a percentage of its full scale, not of the reading. A big dial or a cheap accuracy class can be off by more than the tolerance you are trying to set, and the gauges on inline adjusters often are.
Enter the IP range your regulator needs and the calculator suggests the cheapest accuracy class and the smallest dial that will do, in bar or psi. Or enter the full scale and class of the gauge you already have to see how far off it may be.