I dont know which angle you ar talking about.
The angle of declination is the angle of difference between True North on a map and Magnetic North shown by you compass. Some compasses have an adjustment to tune the compass into True North so you dont have to convert anything. Your Ranger has this adjustment.
The angle of inclination is the difference between The Earth Magnetic field and the surface of the Earth.
On the Equator The magnetic field is close to parallel to the surface. The magnetic field is steep at either pole. If you get a bar magnet and lay some paper over it then sprinkle iron filings over the paper, you will see the lines of magnetism around the magnet. Near the poles of the magnet you will see some lines go straight up and away. Down the sides they will be parallel to the magnet. The poles sort of act as an entry point to the lines of magnetism.
For example if you get a compass needle that is setup for the equator so it sits parallel to the ground. Then you take that compass close to the border of the US and Canada the needle will be tilted down towards the ground on the North side. If you then take that same compass down here to the bottom of Australia it will be tilted down towards the ground on the South side.
Also the Earth is not a perfect sphere, or even in distribution of minerals and land, so the magnetic field bends and curves from that interverence aswell.
The pivot point of the needle will start to bind and stop or inhibit the rotation of the needle in some locations.
Compass manufacturer have tried many different designs over the years, some better than others. The original style Silva (Sweden) base plate compasses came in 5 different needles, Recta made theirs with 2 different needles, one for Northern Hemisphere and 1 for Southern Hemisphere before they come up with their Global Needle system.

So in answer to your question about your Silva working everywhere in the Northern Hemisphere, I dont know but I expect it would work nearly everywhere except close to the North Pole, but you may have to hold it on a bit of an angle in some places.