This is quite an over simplification… Beam width goes in sync with forward gain and sidelobes. The main factor is actually boomlength with a number of elements which are optimally spaced to reach a desired pattern with a desired antenna impedance. I have seen the Rothammel antennabook mentioned before, there is also the ARRL antenna handbook and for example the DK7ZB antennapages. None of this specific to ADS-B but they all contain useful information.
Simply adding or subtracting elements is not the way to go. I believe there are several antenna programs out there which can calculate optimal antenna’s for a specific frequency or band, some including the images as shown earlier. MMANA http://hamsoft.ca/pages/mmana-gal.php , various versions of NEC such as EZNEC https://www.eznec.com/ and antenna magus http://www.antennamagus.com/ come to mind. As I have not yet build any antenna’s myself (yet), I have not used any of these so do not see them as recommendations.