I’m a bit confused about what your asking? How do you quantify strength of heritage? Your maternal grandmother is Mexican. That IS your heritage from this side of your family. You have a heritage from your three other grandparents as well.
I guess the “strength” of your heritage would depend on how strongly YOU identify with it.

If you are asking what percentage of Mexican you are, the answer is ¼

Well, if you’re talking about percentages, your Mexican and your Irish can’t add up to more than 100.

It’s heritage. Her-i-tage.

I agree with both answers above.

You could look at it also as being Native to the Americas if your grandma’s family is Aztec. Similar to the Native Americans in the US. Mexico is made up of many immigrants like the US. My husband is Mexican on both sides, but his grandparents make him ½ Israeli, ¼ Spanish, ¼ French. Unless you go back further his Israeli grandparents, their families had been in Lebanon and we think Syria before Israel. It depends with how far back you identify.

I’m Jewish and my family is mostly from Latvia and Russia. Most Jewish people identify primarily with being Jewish, not with what part of the world their families are from.

It’s really up to you. You might get some benefit by calling yourself Hispanic if there is some sort of quota, but I think you are in California, so I doubt it would help. I am not sure how much “Hispanic” they require to call yourself Hispanic. Was your grandma born in Mexico?

The nifty thing is that heritage isn’t really tied to blood very well. You could be 1/64 Mexican and, so long as you share norms and values and otherwise identify with the Mexican people, you’re Mexican.

Exactly ¼ is unlikely, because if you go back 4,5,6 or more generation you will almost always have a mix of genes. And native Mexicans are actually Asians and all Asians are actually Africans. Ultimately we all are descendants of Africans.