The 'best' conditions for doing this is when you have two angles... add them together and the difference between their total and 180 degrees is the magnitude of the remaining angle. Otherwise, if you...
For any REGULAR n-sided polygon the interior angles are (180 - 360/n) degrees eg for a regular octagon, where n = 8, the calculation is 180 - 360/8 ie 180 - 45 = 135o degrees.