I agree that this is an unnecessary limitation. I usually set -89 to 89 degrees as extreme limits that no one could possibly want (right?) This avoids the divide by zero (TAN(90)) errors without undue restrictions.
You can probably adjust the code pretty easily. If the part is written properly, the limit values should be set in the parameters script. Something like:
VALUES "end_angle" -60, RANGE [-60, 60]
or
VALUES "end_angle" 0, RANGE [0, 60], [300, 360]
(I don't recall if the negative values work in the RANGE function