So I can't exactly find out what angle should the converging part of the rocket chamber be or why different angles are chosen for engines (varies by about 15°).
Are there any guidlines on the angle or equations?
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Have a look at Sutton (7th edition online here) Chapter 3.4
The converging nozzle section between the chamber and the nozzle throat has never been critical in achieving high performance. The subsonic flow in this section can easily be turned at very low pressure drop and any radius, cone angle, wall contour curve, or nozzle inlet shape is satisfactory.