Angle calculation issue with respect to IDF for PTC

Hi,

I'm working on creating an IDF output that will take all place_bounds and export them to an IDF file for better modelling in Pro-E(PTC).

I have a board file with an arc which is identified by the following information from the database (in paranthesis the variables used further below)
arc center = 1.793,0.133 (a_c)
radius 0.058 (r)
startEnd((1.85 0.12) (1.78 0.19))  ((x1 y1) (x2 y2))
unfortunately the data does not contain the arc angle that was specified inside the package symbol so I'll have to calculate this arc angle in degrees.

If I use file, export, IDF and set TYPE=PTC and Version=2.0 then I'll get the following specification/record for this arc
0    1.850    0.120    0
0    1.780    0.190    -244.305
which states that the arc angle is -244.305

If I however uses the cosinusrelations on the data that I have access to (arc center, radius, arc startpoint and arc endpoint) then I get the following
0   1.850000   0.120000  0.00
0   1.780000   0.190000  -242.83
which states that the arc angle is -242.83
This makes a difference for about approximately 1.5 degrees
I'm using the law of cosines to calculate the angle
http://en.wikipedia.org/wiki/Law_of_cosines (remember the angle=result*180/pi -360)

I'm just wondering how this difference occur. Does anyone have an idea to what I could be doing wrong?