when placing a certain type of sundial one needs to point it at true north as apposed to magnetic or the other one.That being said if the meridian line of true north for the location in question is 112 degrees W do you use a compass and adjust 112 degrees W of magnetic north or what?
What you need to find is the magnetic declination for your location.
A very general world map is here which shows the declination in degrees.
There's an online calculator with various ways to enter your location including by zip code or latitude-longitude and elevation or ellipsoidal height if you have it.
Along your meridian, it ranges from 11°E to 15°E in the contiguous US (lower 48 states) depending on the latitude.
There are plenty of places online to describe how to adjust a compass (I can never remember which way to adjust it!). Here's a USGS site.