coordinate system - What level of error expected when using NAD StatePlane CRS from adjacent state?

I am adding Georgia data to my GIS database and, unlike the California data I've added, have decided to use my existing EPSG:3361 (South Carolina StatePlane) coordinate reference system for calculations.

I've done this with reasonable success in North Carolina (South Carolina's northerly neighbor) but never did the research to determine the error suffered by using a linear reference system from a non-intended, albeit adjacent area.

I'm guessing the error shouldn't be more than 50 feet, am I correct in assuming this?

More technical information:

Answer

The northern two-thirds of Georgia should be pretty good, because the South Carolina coordinate reference system is Lambert conformal conic-based. Thus, the standard parallels extend through Georgia too. I ran a point at 31N 85W through the National Geodetic Survey's SPC program to see what the distortion would be. Note: South Carolina's zone is 3900. It returned:

 =======================================================
           Latitude           Longitude      Datum Zone

 INPUT =   N310000.0          W0850000.0     NAD83 3900
 =======================================================

  NORTH(Y)    EAST(X)     AREA  CONVERGENCE   SCALE
  METERS      METERS            DD MM SS.ss   
  ----------- ----------- ----  -----------   ----------
  -85051.112  227348.537  SC    -2 13  3.35   1.00086349

So the distortion is 1.00086349 or 86 parts per 100000 (or 8.6 parts per 10000). A State Plane zone was originally designed to be no worse than 1 part per 10000.