Rich,
You've got the principal:
389+12 means engineering station 389 plus 12 more feet.
The 389 would be 389 chains or 38,900 feet from some arbitrary staring point.
389+12 means 38,912 feet from the starting point.
one mile is 5280 feet so that distance from a starting point would have an engineering station of 52+80
389+12 = 38,912 feet = 7 miles + 1,952 feet
Easy way to figure distances today is just drop the "+" and you have the distance from the starting point. Then you can find the difference (subtract) and get the distance between any two points.
The water pipeline plans and street plans that I work with generally do not start with 0+00, but rather 1+00 or even 10+00. The strating point on these is usually the intersection of tThat way if you have to change the plan start on the far side of an intersection you are never dealing with a negative number (like 0-56) and confuse someone.