You don't need any other tools. Just take the angle between the direction shown by the magic compass and the sun or some other fixed far away landmark, walk some fixed distance perpendicular to the direction of the magic compass and take another measurement, then calculate the other two sides of the triangle using the law of sines. Since MC is a master blacksmith, a protractor or a sextant would be easy to make to take the angles, but even if he can't, you can just eyeball it and get a pretty good estimate anyway. It's not like you need to know the distance to the centimeter.I wonder if classical navigation tools (normal compass, maps,...) are precise enough in this world to use trig to détermine the distance.
They really ought to have done this, since the crow flight path is likely to be very suboptimal for travel. He's supposed to be a genius at dealing with adventuring utility tasks, but apparently he's not THAT clever.You don't even need trig if you have accurate maps and a compus. You can just solve it graphically! Just mark direction from 2 or more points while not moving in the direction of the compus. Then extend the lines till they intersect to get the location.