To edit drop chances go to data/droppablespawneggs/predicates/chance.json
Next change the number after "chance" to the percent chance you want divided by 100 (for example: 100% --> 1.0 | 50% --> 0.5 | 1% --> 0.01 | 0.1% --> 0.001
Default is 0.00005 (0.005% or 1 in 20000)
Now save and type /reload in game