Question それでいい

I have a grammar question, which is probably easy for learners a bit more advanced than me. I hear the sentence in the subject quite often (meaning "That's good.").

I'm confused why the particle で is used here. Assuming it is an adjective sentence with それ being the subject and いい being the predicate, I would have expected a が particle (or maybe は).

What's the right way to look at the sentence grammatically to understand why it has to be で?

I can't explain grammatically and I won't even try because others here can explain better but I would use それがいい to mean "that one (is the one I want)" and それでいい to mean "that one (is good enough)."

Thanks for your answer mdchachi.

I should maybe mention, that it seems to appear often in context of learning some news, so meaning something like "That's good to hear."

What I'm confused about is what kind of usage of で is happening in the sentence. The language is still too new to me to have developed a feel for the particles, and looking it up in various grammar resources hasn't been enlightening either. で usually is described as marking either a location (like に) or meaning "by means of". Neither seems to fit here, so it must be another usage, which I'm not aware of, no?

Not sure if it applies here but sore de is a set phrase which serves as a conjunction "to indicate that what is stated in the preceding sentence is the reason or cause for what is stated in the following sentence."

寿司を食べたくなりました。それで寿司ベントを買った来ました。
I felt like eating sushi. So I bought a sushi bento.

Note, で is also the 'te' version of です which can also be confusing sometimes when you are thinking only of particles.