the levels cover at level 0 30% and at level 1 50% and at level 3 80%
Some mods start at level 1 values at level 0, so you get something like this: level 0 50%, level 1 70% and level 2 90%
Other mods, or other modders have tweaked the values to level 0 65% level 1 85% and level 2 100%.
So for all practical purposes if you can live to the second level you get an invulnerable planet.
Counter for this of course would be to make the shield generator with medium or even low shields or hull points, and have it guarded with perhaps frigates or strikecraft. That way, your invulnerable planet is only invulnerable so long as the generator can stay alive. Which might mean for small or even medium fleets you can defend the shield generator pretty good, but chances in a large fleet attack go down severely. Good way to avoid the small Pirate raiding parties from knocking off your planet early on in the game.
These values are found in the abilityplanetshield.entity's buff as Harpo noted above, and with notepad you can adjust those values yourself (if you have text versions of the entity files in the gameinfo folder in the mod folder)
Take care and have fun!!
-Teal
