This site is not for solicitation of services or 'purchasing' development. Please do not post requesting side mods/plugins and so on. Your thread will be removed, and you will receive a warning.
The "legendary-spawn-ticks" setting there determine the average time between spawn attempt (will vary randomly by up to +/- 40% each time), and "legendary-spawn-chance" sets how likely it is for such an attempt to succeed (1.0 = 100%; defaults to 30%).
-> https://pixelmonmod.com/wiki/Config
Do modify the spawn chances for individual Pokémon, you'll need to create a Data Pack. With that you can override the respective spawn files. The originals are found in the mod file (opened like a ZIP archive) under "data/pixelmon/spawning/" and can be copied into your Data Pack under the same path and edited there.
A lot of aspects play into the effective chance to find a Pokémon though, which can be a bit hard to predict. Since spawn chances are calculated dynamically based on all options available in the conditions the spawn event is happening in, they will not only be affected by their own spawn settings, but indirectly by those of everything else sharing (even partial) conditions as well. And on top of that even much less direct aspects like the exact distribution of terrain, certain abilities, Lures, line of sight, nearby blocks, the highest level on your team, the positions of players and other spawns, and more will have an effect on what exactly will spawn.