How to calculate the angle formed between 2 planets? I am interested in calculating the angles formed between planets like Saturn opposite to Neptune - 180 degrees, PlanetX opposite/trine/square to PlanetY. How to find such degrees?

I use Stellarium software to find positions of planets. Here I am adding positions of Saturn, Jupiter and Sun.







Stellarium shows information about selected objects, can it be used to calculate angle formed, for example, angle formed between Saturn and Jupiter taking Sun as reference point?
The planets are moving along elliptical paths, and we are viewing them from another planet that is also moving along an elliptical path, this makes the motion that the planets make in the sky (relative to distant stars) seem to loop and move with an inconsistent rate. The planets don't all orbit in the same plane, and so may be above or below the ecliptic (the apparent path in the sky of the sun)

So there is no shortcut to finding the angle between two planets: You have to calculate the sky position of the planets and then find the angle between those positions. 

The first step is called calculating an ephemeris. The details are beyond the scope of this answer, but NASA have a good ephemeris calculator at https://ssd.jpl.nasa.gov/horizons.cgi. Alternatively, software like Stellarium will be able to calculate the position of a planet on any given date in the near future or past.

When you have the position of the planets as RA and Dec, and you have converted the units to decimal degrees, you can calculate the angle $A$ between them using

$$\cos(A) = \sin(\mathrm{Dec_1})\sin(\mathrm{Dec_2}) + \cos(\mathrm{Dec_1})\cos(\mathrm{Dec_2})\cos(\mathrm{RA_1} - \mathrm{RA_2})$$

Further details and a calculator is found at http://www.gyes.eu/calculator/calculator_page1.htm